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.
• The script/parse-examples passes without issues.
• There are sufficient tests for the new fix/feature.
• Grammar rules have not been renamed unless absolutely necessary.
• The conflicts section hasn't grown too much.
• The parser size hasn't grown too much (check the value of STATE_COUNT in src/parser.c).

[helixbass]

test/highlight/injection.js was failing, which tests that for the source text:

eval(js `var foo`)

the var foo gets highlighted as JS content (eg var gets highlighted as a keyword)

My understanding is that the reason that test started failing is because the way that injections work, by default this @injection.content doesn't include ("re-parse") any child nodes' content of this (template_string) node. And now there is a (template_chars) child node, so the re-parsing of the (template_string) node doesn't include the var foo content

So this gets that test passing by specifying that the new (template_chars) node should be used as the thing to be re-parsed instead

I have no idea if this is hacky or not, eg not sure how the old version vs this version would behave when highlighting eg a template string with multiple text sections (eval(`is this text parsed as JS ${foo} and then this text is also parsed as JS`))

[amaanq]

Think this should be ok now on 0.20.2