Support string open/content/close in RStringValue

[DavisVaughan]

• Point at tree-sitter-r main after merging Split raw strings into open/content/close r-lib/tree-sitter-r#199

For r-lib/tree-sitter-r#199

No user facing changes.

Main developer facing change is the ability to do this on an RStringValue, and it works correctly for single quoted strings, double quoted strings, and raw strings:

node.open_token()
node.content_token() // optional
node.close_token()

---

Note to self:

Instead of 'string_open' and 'string_close' tokens in the ungrammar, I originally had alternations of ('"' | '\'' | 'raw_string_open') and ('"' | '\'' | 'raw_string_close'). This required tweaks to the codegen xtask (similar to ones we have done in the past, and in line with changes biome has made in the meantime). I did not end up needing this, so I reverted the commit but kept it in the history in case we have to do this in the future, because it is not an obvious fix.