fix(nuxt): generate 200.html and 404.html spa fallback files

[pi0]

🔗 Linked issue

❓ 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

As a regression of nitrojs/nitro#166, we were generating dist/200/index.html instead of dist/200.html for spa fallbacks. This makes expected output.

📝 Checklist

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

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[TheAlexLichter]

This still works only for ssr: false, right?

[pi0]

@manniL Yes 👍 We need nitro route rules as discussed to to selectively set spa mode on these routes when generating with ssr enabled.