Switch to tree-sitter

[gcv]

By switching to tree-sitter, built into Emacs as of version 29, all the questions Snail asks of CSTParser can be answered without leaving Elisp. This will be a significant benefit, not only by eliminating a dependency on CSTParser and its potentially unstable API, but also because Julia startup time will improve.

In general, it'll be great to ditch all Julia-side dependencies for core Snail functionality and leave them to extensions.

[MasonProtter]

By the way, JuliaSyntax.jl is now bundled inside of modern versions of Julia (since 1.9, and the new LTS is going to be v1.10), so you could also just use JuliaSyntax.jl instead of TreeSitter, but if there's other benefits to TreeSitter then I guess we should go with that.