feat: runtime config support for `detectBrowserLanguage`

[BobbieGoede]

🔗 Linked issue

❓ Type of change

• [ ] 📖 Documentation (updates to the documentation or readme)
• [ ] 🐞 Bug fix (a non-breaking change that fixes an issue)
• [x] 👌 Enhancement (improving an existing functionality like performance)
• [x] ✨ New feature (a non-breaking change that adds functionality)
• [ ] ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

• Requires https://github.com/unjs/nitro/pull/1831 to be merged
  • And a nitropack release including this change to be updated in nuxt

These have been merged and released

Primary motivation for adding runtime support is to actually to merge more test files 😂. Most if not all code for detectBrowserLanguage is used at runtime, so there is no need to rebuild the project to test each options.

https://github.com/unjs/nitro/pull/1831 allows us to override runtime configs objects with environment variables, as this is currently not possible.

This PR changes the @nuxt/test-utils code further, startServer now optionally accepts additional environment variables. And I added helper functions to make setting runtime config variables via environment variables easier.

Using the following in a test

  await setRuntimeConfig({
    public: {
      i18n: {
        detectBrowserLanguage: false
      }
    }
  })

Will be converted into NUXT_PUBLIC_I18N_DETECT_BROWSER_LANGUAGE=false and restarts the server with all properties in the passed object.

📝 Checklist

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