Auto-generate PowerShell completion file

[nao1215]

The gup command generates shell completion files for bash, zsh, and fish. However, gup does not generate shell completion file for PowerShell.

I do not have Windows, so this task has a very low priority. I would be happy to have this feature implemented for Windows users!! There is a function (GenPowerShellCompletionFile()) that generates a shell completion file for PowerShell, so it can be implemented if you know where to generate the file.

https://github.com/nao1215/gup/blob/195e47b5a705371af09b3eecdff4c7567fd5de94/internal/completion/completion.go#L17-L26