[feature] shell completion

[Freed-Wu]

Can it provide a completion like https://github.com/cli/cli/blob/cf4b73ff958b272cf3c9c0cf9351459f76b793a0/pkg/cmd/completion/completion.go

https://jmtirado.net/shell-completion-with-cobra-and-go/ may be helpful.

[rhysd]

This would be nice to have though I don't have knowledge to write them.