CLI subcommand support

[pi0]

Support nuxt- and nuxi- external commands same as Nuxt 2. (Current blocker: We need new CLI builder package for shared utils.)

Module builder enhancement: nuxt/module-builder#32

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References:

• Custom CLI Commands rfcs#11
• feat(cli): improvements and external commands nuxt#4314
• Nuxt 2 impl https://github.com/nuxt/nuxt.js/blob/777a4b7f5033c86c37cbd93008f3ca792e4af8bc/packages/cli/src/run.js#L44

[pi0]

Adding from https://github.com/nuxt/framework/discussions/5696, we shall also support local bin/ directory.

[bmulholland]

FYI, this is preventing nuxt/storybook from adding Nuxt 3 support: nuxt-modules/storybook#385

[pi0]

It is comming soon. Please see my comment nuxt-modules/storybook#385 (comment) using citty and nuxt-[name] is recommended.