feat(runtime-config): export `applyEnv` and `getEnv` from nitropack

[danielroe]

🔗 Linked issue

❓ Type of change

• [ ] 📖 Documentation (updates to the documentation, readme, or JSdoc annotations)
• [ ] 🐞 Bug fix (a non-breaking change that fixes an issue)
• [ ] 👌 Enhancement (improving an existing functionality like performance)
• [x] ✨ New feature (a non-breaking change that adds functionality)
• [ ] 🧹 Chore (updates to the build process or auxiliary tools and libraries)
• [ ] ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

This adds applyEnv and getEnv as exports, as well as allowing them to operate on a custom env object instead of reading from process.env.

This would be a useful feature for frameworks to provide more accurate runtimeConfig at the build time. For example, we already inline these utils in https://github.com/nuxt/test-utils/pull/655 and would need to do the same for https://github.com/nuxt/nuxt/issues/24224 and https://github.com/nuxt/nuxt/issues/26960.

Of course, that would be fine, but it would nicer to be able to simulate the runtime behaviour of Nitro with these utils, as suggested in https://github.com/nuxt/nuxt/issues/24224#issuecomment-1805495557.

I've included them in the core nitropack export but I would almost prefer nitropack/utils, perhaps, to allow space for other similar utilities?

📝 Checklist

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