fix(nitro, nuxi): add runtimeConfig types (for #config and useRuntimeConfig())

[danielroe]

🔗 Linked issue

resolves nuxt/nuxt#12837

❓ 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:

• adds support for shimming #config in Nitro routes
• adds initial docs for useRuntimeConfig()
• adds support for typing privateRuntimeConfig & publicRuntimeConfig

Note, the way I've implemented this allows us in future to automatically type runtime config.

📝 Checklist

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

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[tobiasdiez]

I hope it's okay to use this PR for a related question: how does one use #config in jest tests? If one of the tests (indirectly) tries to import #config then this obviously fails without any further setup.

[danielroe]

@tobiasdiez you would likely want to use moduleNameMapper to point it to a stub.