fix(prerenderer): write responses with json signature to original path

[pi0]

🔗 Linked issue

❓ Type of change

• 📖 Documentation (updates to the documentation, readme, or JSdoc annotations)
• 🐞 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)
• 🧹 Chore (updates to the build process or auxiliary tools and libraries)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

See #1962 and nuxt/nuxt#24487 for some context

When prerendering, a route without .html extension (like /foo or /foo.json, etc) could contain a JSON response (wrongly because returned by h3 handler or a SSR renderer).

Nitro were trusting purely on content-type to detect HTML routes and write /foo to public/foo/index.html.

With this PR, if response contents of /foo contains a JSON signature, we write it to public/foo as-is.

📝 Checklist

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