fix(nuxt,schema): merge custom and resolved app configs

[danielroe]

🔗 Linked issue

#15592

❓ 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 fixes a couple of type issues with the current config schema implementation, notably:

1. NuxtOptions/NuxtConfig were defined in conflicting ways - with two different definitions of appConfig - one extending CustomAppConfig and one extending the ResolvedAppConfig. In this PR we now merge the two, smartly choosing the type from CustomAppConfig but picking from ResolvedAppConfig whether or not the item is defined. We also do not include a key signature for CustomAppConfig.
2. It's possible CustomAppConfig was not in fact augmented previously - I've used a different type name for safety.

📝 Checklist

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

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[danielroe]

merging to test on edge - please feel free to comment if you spot any issues 🙏