fix: proper runtimeConfig and publicRuntimeConfig declaration

[AndreyYolkin]

🔗 Linked issue

Resolves #948

❓ 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 backports schema generation approach from https://github.com/nuxt/nuxt/blob/4b2608c74d80551eee857ab74ab470da839894f9/packages/nuxt/src/core/templates.ts#L132, which introduces next fixes/features:

1. Runtime Config (and its public part) now has more accurate types, generated with Untyped
2. $config in Vue templates is declared with the same generated type

📝 Checklist

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

Before:

After:

(Untyped has some struggles with array types, not Nuxt itself issue):

[danielroe]

This is great! I wonder if we ned to do this in a separate template so we can do it conditionally on whether nitro is enabled or not as I think the runtimeConfig has a different format if it's disabled.

[AndreyYolkin]

Wise enough. I'll take another round of refactoring to ensure it has support for nitro-less build

[danielroe]

(converting to draft just for more obvious triage - feel free to mark ready any time you feel like it)

[AndreyYolkin]

I don't really know about this: we can use

const { $config } = useNuxtApp()

const { nuxt2Context: { $config } } = useNuxtApp()

but displayed types will be different

[wattanx]

Nice! Looks good to me!

[danielroe]

Beautiful work! In future we likely need to add a type test and confirm that useRuntimeConfig() works properly (right now it seems to be shadowed by Nitro auto-import).