fix(schema): decrease recursion on DeepPartial

[danielroe]

🔗 Linked issue

resolvesnuxt/nuxt#13838

❓ 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)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

This slightly improves performance of the recursive DeepPartial type. I'm not sure why we had the union here, but removing it does improve the performance of defineNuxtConfig.

We could also allow an escape hatch from DeepPartial with a complementary utility type - might be worth considering if this happens again. UserConfig here is already partial, and doesn't need to be processed.

📝 Checklist

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

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[pi0]

Nice fix!