feat(nuxt): allow accessing NuxtLayout ref via layoutRef

[huang-julien]

🔗 Linked issue

❓ 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 allow the same thing as #19403 but for NuxtLayout : allow using ref .
However this ones use a prop exposed to tranfer data since the LayoutLoader's setup is async.
todo:

• manual test for memory leaks
• fix tests

📝 Checklist

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

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[huang-julien]

@danielroe It seems that this also solves another issue. On the current main branch, this line is not triggered
And this test should fail since with-props layout does render a Fragment or multiple root nodes.
I fixed the CI by setting a single root node within with-props.vue, should I add another test to make sure we get a warning if a layout has multiple root nodes (or is this not needed) ?

[danielroe]

Added a comment here that is also relevant, I think: #19403 (review)

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