chore: setup vite-plus

[hi-ogawa]

I did vp migrate, but then reverted overrides for vite -> vite-plus and vitest -> vite-plus.

Pre-requisite

Install vp cli globally by following https://viteplus.dev/guide/.

Summary

• vp replaces oxfmt (now vp fmt) and tsdown (now vp pack). It also sets up commit hooks for vp fmt. Configuration is in vite.config.ts
• updates README.md for quick gotchas
• updates ci.yml to use vp

Postponed

For now, I postponed setting up things that I'm not too familiar.

• Node version manager: vp support pinning local .node-version, but personally, I'd rather switch global version on my own for testing sake.
• Linting: we didn't have lint setup and I'm not familiar with it.
• Vite / Vitest overrides: We cannot probably do this since we want to coordinate with our own Vitest version for iteration.

[hi-ogawa]

Drive-by fix of a bug / typo in aria template tests.