fix: export runtime types directly from nitropack

[danielroe]

❓ 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

Otherwise users wanting to use a type like NitroApp or NitroAppPlugin would have to do something like:

import { NitroApp } from 'nitropack/dist/runtime/app'

...or use an internal nitro alias.

📝 Checklist

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