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

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