FR: add "completion" subcommand

[pataquets]

Add a completion subcommand which would output an appropriate shell script to provide shell command completion.

No Rust programmer here, but looks like clap has already such a feature 🚀.

[KAGEYAM4]

Not sure this is related but how to get completion similar to watch command. For example ->

• watch systemctl status <tab-completion> shows completion for systemctl command.
  but hwatch systemctl status <tab-completion> dosen't work.

https://github.com/scop/bash-completion/blob/main/completions/watch can be used for reference.