Expose template string chars as nodes

[helixbass]

Fixes #275

This exposes template_chars as a node (so that template string text contents are visible in "leaf" nodes)

Checklist:

• [ ] All tests pass in CI.
• [x] The script/parse-examples passes without issues.
• [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]

Think this should be ok now on 0.20.2