refactor: extract prepareOutDir as a plugin

[sapphi-red]

Description

Extracted prepareOutDir related code as a plugin so that it is handled consistently in both normal build and watch build.

Previously,

• in normal build, prepareOutDir was called after buildEnd and before outputOptions
• in watch build, prepareOutDir was called before options hook

With this PR, prepareOutDir will be called in the first renderStart hook, which is after outputOptions, for both normal build and watch build.

[sapphi-red]

/ecosystem-ci run

[pkg-pr-new]

Open in StackBlitz

npm i https://pkg.pr.new/vite@20373

commit: 8bf8516

[vite-ecosystem-ci]

📝 Ran ecosystem CI on 8bf8516: Open

suite	result	latest scheduled
histoire	❌ failure	❌ failure
vike	❌ failure	❌ failure
react-router	❌ failure	❌ failure
waku	✅ success	❌ failure
vite-plugin-rsc	✅ success	undefined undefined

✅ nuxt, one, astro, storybook, ladle, quasar, sveltekit, analogjs, vite-environment-examples, marko, unocss, qwik, vite-plugin-react, vite-plugin-svelte, vite-plugin-vue, laravel, vitepress, vuepress, vite-setup-catalogue, rakkas, vite-plugin-pwa, vitest, vite-plugin-cloudflare