add comment node that contains plain comments content

[mrnossiom]

Hi 👋🏻,

I'm working on https://github.com/mrnossiom/lspelling that use TS to extract strings and identifiers from source code to spellcheck them. The grammar works fine to extract doc_comments content, string literals content.

This PR adds a comment field on line_comment and block_comment that contain a comment node with the comment content. It enables easy access to comment content without having to do node matching and strip comment markers based on the node type.

This change matches other parts of the grammar, like string_content or doc_comment.
Names for both the node and the field feel a bit too generic to me. Maybe node: comment_content and content?

I intend to propose such a change on other grammars as well. I was wondering if such a change would have a significant perf impact.