feat(RadioGroup): add selected to label slot props

[simoncdn]

🔗 Linked issue

Resolves #1563

❓ 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

The idea is to create a binding name 'selected' that returns a boolean if a radio is selected or not.
So I've created a new function named: guessOptionSelected(option: any) and passed it as a value of the key "selected" in the return of normalizeOptions function.
Then I've created a bind 'selected: option.selected' in the slot.

Code example:

Video example:
https://github.com/nuxt/ui/assets/105924355/e6a07932-a7d7-4fbf-bcba-c415fff1beb8

📝 Checklist

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