feat(nuxt3)!: enable using <NuxtLayout> without pages integration

[danielroe]

🔗 Linked issue

resolves nuxt/nuxt#12829

❓ 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

This PR extracts the Nuxt layouts integration out of the pages module, meaning <NuxtLayout> can be used anywhere in a Nuxt app even if the ~/pages integration isn't.

It also adds support for extends with layouts.

BREAKING CHANGE:

• we remove the pages:layouts:extend hook

Possible improvements

• type the layout prop within <NuxtLayout> - this would require a new utility interface exported from #app that we could hook into here and in PageMeta

👉 Migration

• If you are using pages:layouts:extend hook, then you can switch to using app:resolve, where you will be able to access app.layouts.

📝 Checklist

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

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[pi0]

Maybe split into simple layout usage and a dynamic example for more advanced cases using a computed?

[danielroe]

I'll simplify this. Maybe we can keep the example at the end of the file for the computed.