Discuss: @raw has long been broken and will be removed

[Edward K. Ream]

While working on #2276 (configurable section delimiters), coverage tests revealed that @raw and @end_rave have probably not worked since Leo 5.8, that is, August of 2018!

I can't be sure, but it looks like unitTest.leo never effectively tested @raw.

Happily, @section-delims comes along at just the right time, so there is no longer any need for @raw.

Removing @raw from the read logic is easy. Removing @raw from the write logic will also be straightforward, but I shall not do so until new unit tests cover all the methods in the 'Writing helpers' section of the AtFile class.

Summary

@section-delims will replace the @raw and @end_raw directives, which have apparently not worked since 2018. These changes affect only leoAtFile.py.

All comments welcome.

Edward