Expose string fragments in template string types

[guillaumebrunerie]

This pull request exposes _template_chars as string_fragment in template string types. The same change was made for regular template strings in Javascript recently, which made it much easier to do syntax highlighting for them, so it makes sense to do it for template string types too.

Checklist

• [X] All tests pass in CI.
• [X] There are sufficient tests for the new fix/feature.
• [X] Grammar rules have not been renamed unless absolutely necessary.
• [X] The conflicts section hasn't grown too much.
• [X] The parser size hasn't grown too much (check the value of STATE_COUNT in src/parser.c).

[amaanq]

oops, this slipped past me, thank you

[amaanq]

forgot to merge, thanks again!