Support shell completion for nushell

[LoicRiegel]

Hi, I'm a contributor of the nushell project, and I think it would be really nice if picocli was able to generate shells completions for nushell, just like for bash or zsh. You can have a look at what custom completions for nushell look like in the nu_script repo, where those cusotm completions were added by the community.

I won't be able to implement this, however; I can myself available for any questions and support regarding nu.