feat: add env-flag for env-specific config (#357)

[tillsanders]

This PR is proposing to add a new --env [name] attribute to Nuxi. The attribute will be passed to the envName option of the c12 library. This adds the ability to support environment overrides (environment-specific configuration) for custom environments. Currently, c12 supports that and the types of the nuxt configuration file are aware of that, but there is no way to actually use custom environments, because NODE_ENV cannot be set to values other than production or development. This PR closes the gap, making deployments to different environments easier.

See #357.

Should this be approved, I would be happy to amend the documentation accordingly.

[MickL]

Does this PR also apply for the environment overrides set in nuxt.config.ts? So if I set --env preview it will pick up the second one?

export default defineNuxtConfig({
   $production: {
       ignore: ['app/pages/dev'],
       scripts: {
         registry: {
           cloudflareWebAnalytics: true,
         },
       },
   },
   $preview: {
        ignore: ['app/pages/dev'],
   },
});

[pi0]

For more clarity i would suggest to name it envName / --env-name.

--env might also be usable for other purposes in the future.

[pi0]

PS: i think we can make a shared object similar to how legacyRootDirArgs now i guess since this is arg is shared for all commands.

[tillsanders]

Thank you, I appreciate your feedback! I made the necessary changes as you suggested. I noticed that there is a slight inconsistency in the way the flags are named. Some use kebab-case, others lowerCamelCase – e.g. --logLevel and --no-fork. I decided to go with lowerCamelCase as that seems to be the majority and matches the naming in the loadNuxt() function.

I also made a small amendment to the documentation, here: nuxt/nuxt#28909

Please let me know if any other changes are required – looking forward to have this available!