fix(nuxt): mark non-augmented NuxtApp properties as unknown

[danielroe]

🔗 Linked issue

resolves #14495

❓ 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)
• 🧹 Chore (updates to the build process or auxiliary tools and libraries)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

This adds more typings for internal NuxtApp properties, primarily for our own internal use. None of the properties marked as @internal should be relied on by end users.

It also updates the default typing of unknown properties on NuxtApp to unknown meaning it will properly respect end users' strictness settings and flag issues when the type is missing (for example, if a provided property is renamed).

📝 Checklist

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

[codesandbox]

  Open in CodeSandbox Web Editor | VS Code | VS Code Insiders