feat: add support for observing selected carousel slide index

[itspolly]

🔗 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)
• ✨ 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 change adds a new event emitter to the Carousel component that emits the index of the currently selected slide. This change is required to make carousels support reactivity: i.e, passing state from one carousel to another.

📝 Checklist

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

[itspolly]

I tried to achieve this functionality without this change by adding event listeners to the emblaApi prop (i used a reference to access an instance of the Carousel component in the setup script) but that didn't work.

[pkg-pr-new]

npm i https://pkg.pr.new/@nuxt/ui@3678

commit: dd441fe