feat: expose `__unenv__` flag for unenv classes and functions

[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

it is commonly required to identify if something is coming from unenv or not specially for classes and mocked objects.

This PR adds __unenv__ property to class (instances) and functions to identify.

Note: Since we were already using req.__unenv__ for another purpose (https://github.com/unjs/unenv/issues/72), the type for IncommingMessage and Readable classes is unknown/object.

📝 Checklist

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