Add shell completions for plugins

[AlexLov]

Current implementation of nilla shell completions does not include plugins as available commands that nilla can execute. And it might be a little difficult for end users to find out what plugins are available/installed.
It would be nice to at least include plugins into shell completions so they're visible as available top level commands there.
Even nicer is to also evaluate <plugin> completion <shell> command so they could be added to nilla's shell completions as well.

Anyway, this is just a "nice to have" request. I don't know rust enough to implement it myself (maybe one day I'll find time for this, though 😃).

[TheCodedProf]

This has been discussed and it is planned, however it's yet to be implemented, I've been busy with work but I'm hoping to be able to start work on this over the weekend.