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'],
   },
});

[tillsanders]

I added a commit to address the feedback above. Is there anything else I can do to improve this pull request?

[pi0]

other than usage, looks good to me