Fixes#20416 for Oxlint
- Same `VP_VERSION` gating for config discovery, LSP file watchers, and JS config loading
- Refactor JS config loader into `resolveJsConfig` / `resolveVitePlusConfig` with shared `importConfig` + `loadConfigs`
- Split test fixtures into `vite_config_*` (non VP mode) and `vite_plus_*` (VP mode)
- Add `env` field to fixture `options.json` for passing env vars to test runs
- Fix repo `oxlintrc.json` ignore glob (`*/files/**` → `**/files/**`) for nested fixture paths like monorepo usecase