refactor: use mergeWithDefaults for experimental option

[sapphi-red]

Description

Use mergeWithDefaults for experimental option to dedupe the default values.
Also make ResolvedConfig.experimental a bit more strict.

[sapphi-red]

/ecosystem-ci run

[pkg-pr-new]

Open in StackBlitz

npm i https://pkg.pr.new/vite@20012

commit: e41bce0

[sapphi-red]

/ecosystem-ci run

[vite-ecosystem-ci]

📝 Ran ecosystem CI on e41bce0: Open

suite	result	latest scheduled
histoire	❌ failure	✅ success
analogjs	❌ failure	❌ failure
laravel	❌ failure	❌ failure
previewjs	❌ failure	✅ success
nuxt	❌ failure	❌ failure
qwik	❌ failure	✅ success
vike	❌ failure	✅ success
vitest	❌ failure	❌ failure
waku	❌ failure	✅ success

✅ astro, ladle, quasar, marko, rakkas, react-router, storybook, sveltekit, vite-plugin-pwa, unocss, vite-plugin-svelte, vuepress, vitepress, vite-plugin-react, vite-plugin-vue, vite-plugin-cloudflare, vite-environment-examples, vite-setup-catalogue

[sapphi-red]

The ecosystem-ci failures are not related to this PR.

[sapphi-red]

Merging as I think this isn't controversial.