Generate Powershell completion scripts

[remkop]

Generate Powershell completion scripts from a CommandSpec model.

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

[remkop]

This is a duplicate of #922.