fix(nuxt): improved typing support for app config

[danielroe]

🔗 Linked issue

closes #20440

❓ 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 PR bundles a few type fixes for app config:

1. First, it ensures that even keys that are not actually present in appConfig are represented in its types. This might affect modules that use custom schema. (resolves error TS2339: Property 'nuxtIcon' does not exist on type 'AppConfig' nuxt-modules/icon#62)
2. Second, it ensures that keys can be set in defineAppConfig that are not present on AppConfigInput (resolves nuxt-icon breaks type extension of AppConfigInput? nuxt-modules/icon#72).

I've added a test to cover both.

📝 Checklist

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

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[pi0]

💯