Provide completion script for nr

[sevenc-nanashi]

Clear and concise description of the problem

It would be great if @antfu/ni provides a autocomplete script for nr, like npm run does:

Suggested solution

Make nr support --completion flag (I guess no package manager will add --completion to npm run), which emits completion scripts.

Alternative

No response

Additional context

No response

Validations

• Follow our Code of Conduct
• Read the Contributing Guide.
• Check that there isn't already an issue that request the same feature to avoid creating a duplicate.

[antfu]

PR welcome :)