gh-118235: Move RAISE_SYNTAX_ERROR actions to invalid rules and make sure they stay there (GH-119731)

The Full Grammar specification in the docs omits rule actions, so grammar rules that raise a syntax error looked like valid syntax.
This was solved in ef940de by hiding those rules in the custom syntax highlighter.

This moves all syntax-error alternatives to invalid rules, adds a validator that ensures that actions containing RAISE_SYNTAX_ERROR are in invalid rules, and reverts the syntax highlighter hack.