ocaml: Improve highlighting and bracketing (#20700)

Some small improvements to OCaml. Would happily split these into smaller
changes, discard anything, etc.

Before:
<img width="441" alt="before"
src="https://github.com/user-attachments/assets/2fb82b03-0d45-4c59-a94d-6f48d634fe19">

After:
<img width="448" alt="after"
src="https://github.com/user-attachments/assets/aa232d8f-4b1b-48f8-93e2-2147de37a20d">

OCaml highlighting and bracketing improvements
    - Fixed bug where `<` was automatically closed with `>`.
    - Typing `{|` now automatically closes with `|}`
- Type variables are now colored with `variable.special` instead of
`variable`.
- Argument names in function declarations and application are now
colored with `label` instead of `property`, even if they are punned.
- `[@@` and `[%` in macros are now colored as bracket punctuation to
match the closing `]`, rather than colored as `attribute`

Release Notes:

- N/A