fix(prerender): add extension to prerendered queries

[farnabaz]

🔗 Linked issue

resolves#1454
resolves#1281

❓ 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

Add .json extension to pre-rendered queries.
The issue here is that npx serve with -s option, redirects all URLs (without extension) to index.html and therefore serving generated websites with nuxi preview return 404 for API routes.

In current state, npx nuxi preview and npx serve -s .output/public does not work and only npx serve .output/public works properly.

📝 Checklist

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

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[pi0]

LGTM 👍🏼 Nitro should handle custom extensions. Only please double check if it does grab those URLs to prerender.