Nice. Some minor details I noticed:
#1 c.syn -- Missing keyword "constexpr"
#2 You have no synhi file for IDL. See attached. Based on years of actual usage :P as well as definitive sources including
#3 No synhi for markdown (.md). Here you go (see below)
#4 No synhi for Powershell (.ps1 .psm .psm1). Here you go (see below)
:-)
----idl.txt------------------------------------------------------------
wordset=-0-9A-Z_a-z
HiliteIncompleteQuotes=TRUE
IgnoreCaseOfKeyWords=FALSE
HiliteIf0StyleComments=FALSE
Transparent=TRUE
CursorLineFlag=CursorLineOverrides
NumberFlag=C-Hex
MultiLineDelimited1=/* */
TillEOL1=//
Quote1="
Quote2='
QuoteEscape1=\
QuoteEscape2=\
Directive1=#
[KeyWords1]
activatable aggregatable agile align allocate allowforweb allowmultiple
annotation apicontract appobject async async_uuid attribute attributename
attributeusage auto auto_handle bem_handle bindable boolean both broadcast
bstring byte byte_count callback call_as case cdecl char coclass code
comm_status composable const constructor_name contentproperty context_handle
context_handle_noserialize context_handle_serialize contract contractversion
control cpp_quote cs_char cs_drtag cs_rtag cs_stag cs_tag_rtn custom
custom_allocator declare declare_guid decode default defaultbind
defaultcollelem defaultvalue defaultvtable default_allocator
default_interface default_overload delegate deprecate deprecated
disable_consistency_check dispinterface displaybind dllname double dual
enable_allocate encapsulated_union encode endpoint entry enum event eventadd
eventremove exclusiveto experimental explicit_handle extension extern FALSE
far fault_status feature feature_name first_is flags float force_allocate
from_contract funcdescattr get handle handle_t heap helpcontext helpfile
helpstring helpstringcontext helpstringdll hidden hostContract hyper id
idempotent idldescattr ignore iid_is immediatebind implicit_handle import
importlib in include inline input_sync int interface interface_name internal
interpret in_line ISO_LATIN_1 ISO_MULTI_LINGUAL ISO_UCS last_is lcid
length_is library licensed local local_call long long_enum manual
marshaling_behavior max_is maybe message methods method_name midl_pragma
midl_user_allocate midl_user_free min_is module ms_conf_struct ms_union mta
namespace ncalrpc near nocode nointerpret non-idempotent nonbrowsable
noncreatable none nonextensible notify notify_flag NULL object odl off_line
oleautomation optimize optional optional_interface out out_of_line overload
overridable overridable_name partial partial_ignore pascal pipe
pointer_default pragma predeclid private_char_16 private_char_8 properties
propget propput propputref protected protected_name proxy ptr public range
readonly ref register remote_async remote_sync remove replaceable
represent_as requestedit requires restricted return_name retval runtimeclass
SAFEARRAY set shape short short_enum sh_composition sh_event sh_file sh_job
sh_mutex sh_pipe sh_process sh_reg_key sh_section sh_semaphore sh_socket
sh_thread sh_token signed sizeof size_is small source sta standard static
static_name stdcall strict_context_handle string struct switch switch_is
switch_type system_handle target_all target_delegate target_enum target_event
target_field target_interface target_method target_parameter target_property
target_runtimeclass target_runtimeclass_member target_struct threading
transmit_as TRUE type typedef typedescattr type_strict_context_handle
uidefault unaligned union unique unsealed unsigned user_marshal user_marshall
usesgetlasterror uuid v1_array v1_enum v1_string v1_struct vararg vardescattr
VARIANT_FALSE VARIANT_TRUE version void volatile winrt_remote_async
wire_marshal wire_marshall zeroout_alignmentgap _asm _cdecl _declspec _export
_far _fastcall _fortran _huge _inline _loadds _near _pascal _saveregs
_segment _self _stdcall __alignof __asm __cdecl __declspec __export
__external_symbol __far __fastcall __float128 __float80 __fortran __huge
__inline __int128 __int32 __int3264 __int64 __interface_interception_info
__loadds __method_property __near __pascal __ptr32 __ptr64 __saveregs
__segment __self __stdcall __unaligned __w64 __winrt_type_serialization_info
[KeyWords2]
( ) * - [ ]
[KeyWords3]
AsyncActionCompletedHandler AsyncActionProgressHandler AsyncActionWithProgressCompletedHandler AsyncOperationCompletedHandler AsyncOperationProgressHandler AsyncOperationwithProgressCompletedHandler AsyncStatus BSTR Boolean Char16 CollectionChange DateTime DeferralCompltedHandler Double EventHandler EventRegistrationToken Guid GuidHelper HRESULT IAsyncAction IAsyncActionWithProgress IAsyncInfo IAsyncOperation IAsyncOperationWithProgress IClosable ICreateErrorInfo IDispatch IErrorInfo IGetActivationFactory IInspectable IIterable IIterator IKeyValuePair IMap IMapChangedEventArgs IMapView IMemoryBuffer IMemoryBufferReference IObservableMap IObservableVector IPropertySet IPropertyValue IReference IReferenceArray IStorage IStorageFile IStorageFolder IStorageItem IStream IStringable ISupportErrorInfo ITypeInfo ITypeLib IUnknown IVector IVectorView IWwwFormUrlDecoderEntry Int16 Int32 Int64 MapChangedEventHandler MemoryBuffer Point PropertySet PropertyType PropertyValue Rect Single Size StorageFolder String StringMap TimeSpan TypedEventHandler UInt16 UInt32 UInt64 UInt8 ULONG USHORT Uri VARIANT VARIANT_BOOL VARTYPE ValueSet VectorChangedEventHandler WwwFormUrlDecoder WwwFormUrlDecoderEntry __int64 boolean byte char double error_status_t float handle_t hyper int long short signed small unsigned void wchar_t
[KeyWords4]
cpp_quote
----markdown.txt------------------------------------------------------------
wordset=-0-9A-Z_a-z
HiliteIncompleteQuotes=FALSE
IgnoreCaseOfKeyWords=TRUE
HiliteIf0StyleComments=FALSE
Transparent=TRUE
CursorLineFlag=CursorLineOverrides
NumberFlag=0
MultiLineDelimited1=<!-- -->
MultiLineDelimited2=<comment> </comment>
Quote1="
QuoteEscape1=\
[KeyWords1]
!doctype </style <style a above address applet array b base basefont bgsound
big blink blockquote body box br caption center cite code comment dd dfn dir
div dl dt em embed fig font form frame frameset h h1 h2 h3 h4 h5 h6 head hr
html i img input isindex kbd li map marquee menu meta nextid nobr noframes
object ol option p param pre range root samp script select small sound sqrt
strike strong style sub sup table tbody td textarea tfoot th thead title tr
tt u ul var wbr xmp
[KeyWords3]
action align alink alt background behavior below bgcolor border bordercolor
bordercolordark bordercolorlight cellpadding cellspacing class classid clear
codebase color cols colspan compact content dingbat direction dynsrc face
height href hspace http-equiv id ismap language leftmargin link loop method
name onload plain rows rowspan size src start target text topmargin type
valign value vlink vspace width wrap
[KeyWords4]
! # ( ) * + --- . === > [ \ ] _ ` { }
[KeyWords5]
á â ´ æ à & å ã ä
¦ ç ¸ ¢ © ¤ ° ÷ é
ê è &emdash;   &endash;   ð ë ½
¼ ¾ > í î ¡ ì ¿ ï
« < ¯ µ · ¬ ñ ó ô
ò ª º ø õ ö ¶ ± £
" » ® § ­ ¹ ² ³ ß þ
× ™ ú û ù ¨ ü ý ¥ ÿ
‍ ‌
----ps1.txt------------------------------------------------------------
wordset=-0-9A-Z_a-z
HiliteIncompleteQuotes=TRUE
IgnoreCaseOfKeyWords=TRUE
HiliteIf0StyleComments=TRUE
Transparent=TRUE
CursorLineFlag=TransparentCursorLine
NumberFlag=Integer|C-Hex|ASM-Hex
TillEOL1=#
Quote1="
Quote2='
Directive1=#
[KeyWords1]
% ? ac asnp cat cd chdir clc clear cli clp cls clv copy cp cpi cpp cvpa del
diff dir echo epal epcsv erase fc fl foreach ft fw gal gc gci gcm gdr ghy gi
gl gm gp gps group gsnp gsv gu gv gwmi h history iex ihy ii ipal ipcsv kill
lp ls mi mount move mp mv nal ndr ni nv oh popd ps pushd pwd r rd rdr ren ri
rm rmdir rni rnp rp rsnp rv rvpa sal sasv sc select set si sl sleep sort sp
spps spsv sv tee type where write
[KeyWords2]
& ( ) * + , - / : < = > [ ] ^ | ~
[KeyWords3]
add-content add-history add-member add-pssnapin clear-content clear-item
clear-itemproperty clear-variable compare-object convert-path
convertfrom-securestring convertto-html convertto-securestring copy-item
copy-itemproperty export-alias export-clixml export-console export-csv
foreach-object format-custom format-list format-table format-wide get-acl
get-alias get-authenticodesignature get-childitem get-command get-content
get-credential get-culture get-date get-eventlog get-executionpolicy get-help
get-history get-host get-item get-itemproperty get-location get-member
get-pfxcertificate get-process get-psdrive get-psprovider get-pssnapin
get-service get-tracesource get-uiculture get-unique get-variable
get-wmiobject group-object import-alias import-clixml import-csv
invoke-expression invoke-history invoke-item join-path measure-command
measure-object move-item move-itemproperty new-alias new-item
new-itemproperty new-object new-psdrive new-service new-timespan new-variable
out-default out-file out-host out-null out-printer out-string pop-location
push-location read-host remove-item remove-itemproperty remove-psdrive
remove-pssnapin remove-variable rename-item rename-itemproperty resolve-path
restart-service resume-service select-object select-string set-acl set-alias
set-authenticodesignature set-content set-date set-executionpolicy set-item
set-itemproperty set-location set-psdebug set-service set-tracesource
set-variable sort-object split-path start-service start-sleep
start-transcript stop-process stop-service stop-transcript suspend-service
tee-object test-path trace-command update-formatdata update-typedata
where-object write-debug write-error write-host write-output write-progress
write-verbose write-warning
[KeyWords4]
alias certificate environment filesystem function registry variable
[KeyWords5]
about_alias about_arithmetic_operators about_array about_assignment_operators
about_associative_array about_automatic_variables about_break
about_command_search about_command_syntax about_commonparameters
about_comparison_operators about_continue about_core_commands
about_display.xml about_environment_variable about_escape_character
about_execution_environment about_filter about_flow_control about_for
about_foreach about_function about_globbing about_history about_if
about_line_editing about_location about_logical_operator about_method
about_namespace about_object about_operator about_parameter about_parsing
about_path_syntax about_pipeline about_property about_provider
about_pssnapins about_quoting_rules about_redirection about_ref
about_regular_expression about_reserved_words about_scope about_script_block
about_shell_variable about_signing about_special_characters about_switch
about_system_state about_types about_where about_while about_wildcard default