Support PowerShell completion

[remkop]

PowerShell supports customizable TAB completion. It should be possible to generate a custom argument completion from a picocli model.

See:

• https://docs.microsoft.com/en-us/powershell/module/microsoft.powershell.core/register-argumentcompleter?view=powershell-7
• https://docs.microsoft.com/en-us/powershell/module/microsoft.powershell.core/about/about_functions_advanced_parameters?view=powershell-7
• Example: https://github.com/click-contrib/click-completion
• Example: https://github.com/clap-rs/clap/blob/master/clap_generate/src/generators/shells/powershell.rs