fix(schema): add back PublicRuntimeConfig interface for augmentation

[danielroe]

🔗 Linked issue

resolvesnuxt/nuxt#13808

❓ 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 PR restores the PublicRuntimeConfig interface, as a vehicle for augmenting types for runtimeConfig['public'] which otherwise can't really be done. (Otherwise each augmentation would completely override the type of public.)

Particularly value your input @pi0 as you touched this last.

📝 Checklist

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

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