OCaml tree-sitter support

### Describe your request

It would be nice to have a `+tree-sitter` module flag on `:lang ocaml`, as tuareg-mode's syntax highlighting is quite poor compared to, say, helix (it also seems to ignore your theme). I'm not sure how this would be done as https://github.com/dmitrig/ocaml-ts-mode/ seems to be dead, but I just thought I'd put this request out there.

Thank you to the maintainers for your hard work 🫡.

Start at the :lang ocaml module flag and compare the current tuareg-mode setup with the proposed ocaml-ts-mode integration linked in the issue. Determine whether a maintained OCaml tree-sitter mode can be used and how +tree-sitter should enable it. Done means :lang ocaml +tree-sitter provides tree-sitter-based syntax highlighting without disrupting the existing setup.

Written by the indexing model from the issue text.