feat: defineRenderHandler

[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

Nitro has a special renderer option that registers a fallback route mainly used for server-side-rendering.

New defineRenderHandler can be used in place of h3.defineEventHandler with better context of how to handle ssr routes. Some new possibilities:

• Allow serving asset fallbacks without hitting SSR endpoint (Enable global serve-placeholder for runtime server #94)
• Caching support
• Typed response normalized and sanitized before sending

I'm starting this as bare minimum and for now undocumented to use for nuxt and iterate to improve wrapper.

📝 Checklist

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