fix(netlify): allow writing config.json

[ascorbic]

🔗 Linked issue

Blocks nuxt/image#1287

❓ Type of change

• 📖 Documentation (updates to the documentation, readme, or JSdoc annotations)
• 🐞 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)
• 🧹 Chore (updates to the build process or auxiliary tools and libraries)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

Adds support for writing Netlify config. Currently this only supports configuring image domains, but will eventually allow all configuration options. This is needed to support the domains option in nuxt/image

📝 Checklist

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

[pi0]

(as discussed internally, merging this as fix to land earlier and allow framework support for image config)