fix: augment @nuxt/schema rather than nuxt/schema

[danielroe]

🔗 Linked issue

❓ 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

Context: nuxt/nuxt#28332

nuxt/schema is a re-export of @nuxt/schema for users to use. Modules should not augment it, or it may end up overwriting the inferred types from @nuxt/schema.

(We made the change in nuxt/module-builder#295 (released in v0.8.0 of @nuxt/module-builder) to avoid doing this in @nuxt/module-builder itself.)

📝 Checklist

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