Add improved grammars for Yacc and Lex

[Alhadis]

This PR adds a grammar for Lex/Flex, files, and replaces the one used to highlight Yacc/Bison.

Yacc/Bison

We're still using the old TextMate bundle, which hasn't had an update in 12 years. Furthermore, the bundle was only an unfinished stub, according to the commit notes of textmate/bison.tmbundle@7ffa521:

Commit a bare-bones start at a Bison bundle that I've had sitting around for a while

git-svn-id: http://svn.textmate.org/trunk/Bundles/Bison.tmbundle@7733 dfb7d73b-c2ec-0310-8fea-fb051d288c6d

I've rewritten it from scratch. The improvements are immediately apparent; I doubt this PR will take very long to review. 😜

/cc @pchaigno

Checklist:

• I am changing the source of a syntax highlighting grammar
  • Lex: Preview
  • Yacc: Before | After

[Alhadis]

On a sidenote, I had to use source.c++ as the embedded grammar's scope, instead of source.cpp. The former is used by the old TextMate bundle; the latter by Atom (changed by @maxbrunsfeld in atom/language-c@f74f7b3).

I'd like the language-grammars package to support both GitHub and Atom users, but that would necessitate two different scope-names, which will raise a complaint from our grammar compiler. @vmg, is this something we can avoid?

[pchaigno]

The Lex grammar seems to be having issues "parsing" the middle of the preview file. Is that expected?

[Alhadis]

(I see I was a bit too hasty with selecting a preview file...😂) Yes, that is to be expected, as the file isn't completely valid Lex source. In fact, it almost appears as if it were copied from an HTML rendering of a Lex file's source code... 😕

I'll pick another...

[Alhadis]

I was also using the wrong highlighting grammar (jlex.cson instead of lex.cson), which probably wasn't helping...

[pchaigno]

LGTM!