feat: allow customizing root id and tag

[pi0]

🔗 Linked issue

Resolves nuxt/nuxt#15132 Resolves nuxt/nuxt#15312

❓ 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 customizing root element id and tag:

export default defineNuxtConfig({
  app: {
    rootId: 'app',
    rootTag: 'main'
  }
})

Renders Nuxt app into <main id="app">

📝 Checklist

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

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