feat: Highlight Rust String interpolation macros

[nik-rev]

Hi, so I made a tree sitter grammar for rust format_args! which improves the Helix Rust experience, first of all by making it look prettier and easier to tell text apart from actual syntax

parser repo: tree-sitter-rustfmt: https://github.com/nik-rev/tree-sitter-rustfmt

Here's how it looks like:

Before

After

Notes

At the moment the PR injects this language into every string. What should be done instead is that it's injected into every string within a macro's arguments. I tried to do this, I'm not sure how to exactly. This was my best attempt, but it completely turned off syntax highlighting when I tried it:

(macro_invocation
  (string_content) @injection.content
  (#set! injection.language "rustfmt"))

Closes #5845

[archseer]

I think we need to be smarter here and match on the macro name

[adamchalmers]

I agree, I think just injecting it for stdlib and a few known crate macros (e.g. slog and tracing macros) would be a huge improvement already, and then you could always add more crates over time.