Install cmdliner based completions

[WardBrian]

This is the final patch in the series:

1. Source compatibility with newer cmdliner versions #6854
2. Update cmdliner vendor to 2.1.1 #6867
3. Update argument definitions with cmdliner completion functions #6868

This replaces the existing zsh and bash completion scripts with the ones that leverage cmdliner, and adds a new completion script for powershell (closes #6853)

Question: I am not entirely sure what the difference between SH_pwsh Powershell and SH_pwsh Powershell_pwsh is, but the Register-ArgumentCompleter functionality should be available in all powershell versions after 5.0