fix(nuxt): async transform for inline middleware

[danielroe]

🔗 Linked issue

resolves #18459

❓ 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

We were not transforming inline middleware, meaning that the nuxt instance would be lost after an await.

We might also consider an option to add*Plugin to accept an options argument for unshift/push/etc.

📝 Checklist

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

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[antfu]

I think having options of unshift/push would be great!

[danielroe]

Great - let's do it! For this PR, it doesn't resolve the issue as the unctx plugin needs to be added last; it's not enough to unshift the page-meta plugin, for example.