Feature: Add completion script for powershell

[frostming]

This patch adds an option --posh to pip completion and the completion works perfectly on my machine(Windows 10 and Powershell 5.1.19041.1)

Close #9024

Things need to be polished or discussed:

• I pick a common short name --posh for Powershell, tell me if you prefer the full version: --powershell.
• The completion may work unexpectedly if one adds more than one completion snippets(say python -m pip and pip). It is caused by the fixed name of the backup function. To overcome this, we need a unique name for each prog. As a result, we should feed a second value to the template besides prog. But since the current implementation works for most cases so I would like to ask for maintainers' opinion on this.

[frostming]

/cc @pradyunsg @pfmoore

[uranusjr]

Isn’t pwsh more widely recognised? But I’d prefer spelling out --powershell.

[pfmoore]

I strongly prefer --powershell.

[frostming]

Yes, that makes sense, already updated the option name

[pradyunsg]

@frostming Any chance you could update this PR? :)

[frostming]

@pradyunsg Sorry this has been missing from my timeline for a long time. Already rebased.

[jimbrig]

Any updates on this feature request?

[uranusjr]

I didn’t read the autocompletion code (don’t understand Powershell autocompletion), but otherwise looks good to me.

[pradyunsg]

Thanks @frostming! ^>^