Intent to prototype and ship: CSS generic() function

[Fuqiao Xue]

Summary:
CSS is introducing a new functional syntax for newer generic font
families: `generic()`. Instead of adding more bare keywords that might
clash with real font names, script-specific generics are written like
`generic(fangsong)`, `generic(kai)`, `generic(khmer-mul)`, and
`generic(nastaliq)`. I'd like to prototype and ship the first two
generic() keywrods: `generic(fangsong)` and `generic(kai)`.

Bug:
https://bugzil.la/1868500

Specification:
https://drafts.csswg.org/css-fonts-4/#generic-font-families (initially
in https://github.com/w3c/csswg-drafts/issues/4606 )

Standards Body:
CSS WG

Platform Coverage:
All

Preference:
layout.css.generic-font-family.enabled

DevTools Bug:
N/A

Extensions Bug:
N/A

Use Counter:
N/A

Standards-Positions Discussion:
https://github.com/mozilla/standards-positions/issues/1449

Other Browsers:
Blink: not sure
WebKit: not sure

web-platform-tests:
See the bug.