Add built-in PowerShell shell completion support

[doctorlai-msrc]

Summary

Adds PowerShellComplete as a fourth built-in shell completer, alongside the
existing BashComplete, ZshComplete, and FishComplete. Users of Click-based
CLIs on Windows PowerShell 5.1+ or PowerShell (pwsh) 7+ can now opt into TAB
completion the same way they do for the other shells:

# In $PROFILE
$env:_FOO_BAR_COMPLETE = 'powershell_source'
foo-bar | Out-String | Invoke-Expression
Remove-Item Env:_FOO_BAR_COMPLETE

…or generate the script once and source it:

$env:_FOO_BAR_COMPLETE = 'powershell_source'
foo-bar | Out-File -Encoding utf8 ~/.foo-bar-complete.ps1
Remove-Item Env:_FOO_BAR_COMPLETE
# then in $PROFILE:
. ~/.foo-bar-complete.ps1

PowerShell support has been a long-standing gap in Click; downstream projects
have repeatedly worked around it by shipping ad-hoc completer scripts. This PR
makes it a first-class built-in so every Click-based CLI gets it for free.