Is your feature request related to a problem? Please describe.
Completion for bash and zsh is already supported. Fish and PowerShell completions are missing.

Describe the solution you'd like
There should be a CLI option for generating fish and PowerShell completions, like there is for bash and zsh.

E.g. proji completion fish

Additional context

Proji bash completion: https://github.com/nikoksr/proji/blob/master/cmd/proji/cmd/completionBash.go

Proji zsh completion: https://github.com/nikoksr/proji/blob/master/cmd/proji/cmd/completionZsh.go

Cobra fish docs: https://github.com/spf13/cobra/blob/master/fish_completions.md

Cobra powershell docs: https://github.com/spf13/cobra/blob/master/powershell_completions.md