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

[danielroe]

🔗 Linked issue

❓ Type of change

• [ ] 📖 Documentation (updates to the documentation, readme or JSdoc annotations)
• [x] 🐞 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: https://github.com/nuxt/nuxt/issues/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 https://github.com/nuxt/module-builder/pull/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.