feat: support prerendering binary files

[danielroe]

🔗 Linked issue

#319

❓ Type of change

• [ ] 📖 Documentation (updates to the documentation or readme)
• [ ] 🐞 Bug fix (a non-breaking change that fixes an issue)
• [x] 👌 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 supports writing prerendered responses as binary data directly.

To resolve #319 fully, we also need to support returning binary data in unenv rather than always coercing it to a string. (https://github.com/unjs/unenv/pull/43).

📝 Checklist

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