Fixes#20416 for Oxfmt
- Gate `vite.config.ts` recognition behind `VP_VERSION` env var
- With it, only vite.config.ts is recognized
- Without it, excluded from config candidates (.oxfmtrc.json, oxfmt.config.ts are used)
- Refactor JS config loader into `loadJsConfig` / `loadVitePlusConfig` with shared `importJsConfig` helper
- `loadVitePlusConfig` should be updated to use `resolveConfig` from `vite-plus` soon
- Split tests into `vite_config` (non VP mode) and `vite_plus` (VP mode)