Add a "JSON with Comments" grammar

[savetheclocktower]

Copied from a draft issue on the WASM/Modern tree-sitter project.

This is an interesting idea because tree-sitter-json properly parses JavaScript-style comments. They aren't automatically errors, which means that tree-sitter-json could just as easily support a JSONC grammar (i.e., “JSON with comments,” used in the VS Code sense) as it does a JSON grammar.

Right now, inserting a comment into a JSON file (using its modern Tree-sitter grammar) has no effect; the comment is highlighted neither as a comment nor as an invalid token.

We could do the following:

• Create another grammar called “JSONC” (or “JSON (with comments)” or something similar) that's nearly identical to the JSON grammar. They can share the same tree-sitter-json WASM files.
• Both grammars should use the same existing highlights.scm, but each one should import its own highlights.scm that's unique from the other one:
  • The JSON grammar's unique SCM file would mark comment nodes as invalid.illegal.
  • The JSONC grammar's unique SCM file would mark comment nodes as comment.line or comment.block as appropriate.
• Since some usages of JSON-with-comments seem to reuse the .json file extension, we could also add an “Allow comments in JSON files” setting to the language-json config schema. If that setting is enabled, the JSON grammar's behavior with comments would be reversed.

[savetheclocktower]

There's another draft issue about adding a JSON5 grammar. That would take more effort, since some of JSON5's enhancements aren't understood by tree-sitter-json. There is a community JSON5 tree-sitter parser (though I haven't investigated it too deeply), but since it'd need a different parser, it might as well be a community package unless people clamor for it. (I think JSON5 is a good idea, but it's not exactly catching on in my corners of the development world.)