feat(kit): support config extends using unjs/c12

[pi0]

🔗 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)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

Use unjs/c12 for loading configuration. This allows easier future developments with a well-tested library and share standard config loading mechanism as a standalone utility.

This PR migration adds a new functionality to nuxt.config. With a new extends key, we can extend nested levels of nuxt config or even fetch from remote git source!

// nuxt.config
export default defineNuxtConfig({
  extends: [
     './nuxt.config.dev',
     './base',
    'github:nuxt/theme'
  ]
})

Config layers are available as nuxt.options.layers. Each module can optionally use it for extended merging functionality.

📝 Checklist

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

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