fix(schema): mark schema as free of side effects

[danielroe]

🔗 Linked issue

nuxt/cli#83

❓ 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)
• 🧹 Chore (updates to the build process or auxiliary tools and libraries)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

Discovered when removing @nuxt/schema from externals, it seems rollup inlines various dependencies of @nuxt/schema even when just importing types. We can work around this by marking it as side-effect free, which also happens to be the case.

📝 Checklist

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

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