Add support for oxfmt formatter (#382)

## Summary

- Add oxfmt (Oxc Formatter) as a new formatter option for JavaScript and
TypeScript files

## Details

[Oxfmt](https://oxc.rs/docs/guide/usage/formatter) is a Rust-powered,
Prettier-compatible code formatter from the Oxc project. It's currently
in alpha but offers significant performance improvements (30x+ faster
than Prettier).

Key points:
- Uses `inplace` mode since oxfmt doesn't support stdin/stdout
- Works with all common JS/TS extensions (.js, .jsx, .mjs, .cjs, .ts,
.tsx, .mts)
- Not added to `apheleia-mode-alist` - users can opt-in by setting
`apheleia-formatter` to `oxfmt`
- Defaults are already sensible: 2-space indent, printWidth 100, no tabs

Example usage:
```elisp
(setf (alist-get 'oxfmt apheleia-formatters)
      '("apheleia-npx" "oxfmt" inplace))

;; To use for JS/TS files:
(setq-default apheleia-formatter 'oxfmt)
;; Or per-mode:
(setf (alist-get 'typescript-ts-mode apheleia-mode-alist) 'oxfmt)
```

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