fix: add `extend` method to debug mock

[danielroe]

🔗 Linked issue

https://github.com/nuxt/nuxt/issues/15608

❓ Type of change

• [ ] 📖 Documentation (updates to the documentation or readme)
• [x] 🐞 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

This adds the extend method to our debug mock.

https://github.com/debug-js/debug/blob/d1616622e4d404863c5a98443f755b4006e971dc/src/common.js#L149-L153

📝 Checklist

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