fix(nuxt): allow union type arguments for useAsyncData

[tobyfaux]

🔗 Linked issue

Resolves nuxt/nuxt#14698.

❓ 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)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

Resolves nuxt/nuxt#14698 by updating the KeysOf type so that a union type argument yields all keys of all types in the union. This ensures that calling useAsyncData with a union type argument will result in the returned data value having the same union type instead of a type consisting of only the members that are common to all types in the union.

📝 Checklist

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

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