Add generating shell completions for `os` and `home` plugins

[AlexLov]

It would be really nice to have shell completions for os and home plugins. Unfortunately, I'm not very proficient in Go, so cannot help on it much, but I believe there should be a simple method/package for this.

[arnarg]

I agree that this would be very useful! I'm however not sure how/if we can merge the completions with nilla-cli's. It would be simple to simply generate them for nilla-os and nilla-home with the cli module I'm using otherwise, iirc.

[AlexLov]

Note I also started this discussion on nilla-cli repo about supporting completions for plugins.