feat: allow extending with theme config

[pi0]

🔗 Linked issue

Resolvesnuxt/nuxt#14329

❓ 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 allows extending nuxt app using theme alias config. Typing for theme config in nuxt.config is intentionally set to string while string[] is also possible by c12. This is to guide the correct usage of extends vs theme. Extends from theme has highest layer priority by default too.

📝 Checklist

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

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