feat(nuxt): generate spa fallbacks for nuxt generate

[pi0]

🔗 Linked issue

Resolves nuxt/nuxt#13990

❓ Type of change

• 📖 Documentation (updates to the documentation or readme)
• 🐞 Bug fix (a non-breaking change that fixes an issue)
• 👌 Enhancement (improving an existing functionality like performance)
• ✨ New feature (a non-breaking change that adds functionality)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

This PR generates fallback SPA page for nuxt generate (ssr: true) in order to make fallback supported.

• nuxt: generate + ssr: true (default): 200.html is generated (new) without being server-side rendered.
• nuxt: generate + ssr: false: 200.html, 404.html and index.html (new) are generated

Nitro prerenderer detects this explicit routes and disable SSR for them. To avoid confusing with actually prerendering top level /, index.html is only pre-rendered for ssr: false. Also 404.html is only generated for ssr: false. It can be added for ssr: true using nitro.prerender.routes: ['/404.html'] if needed.

📝 Checklist

• I have linked an issue or discussion.
• I have updated the documentation accordingly.

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