fix: deprecate @unhead/schema

[harlan-zw]

🔗 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)
• ✨ 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

To simplify the package we look to minimise the dependency graph, starting by having unhead itself export the schema instead of the separate package.

We also inline the zhead dependency into /types/schema.

The @unhead/schema package is now simply an alias for unhead/types.