Support string gaps, raw strings, etc.

[414owen]

It looks like these aren't currently implemented?
See https://web.archive.org/web/20250619083120/https://lean-lang.org/blog/2024-2-4-lean-450/

[Julian]

There are definitely a huge number of things not implemented yet, essentially because of tree-sitter performance (size) issues which put this work on hold. PRs certainly welcome.