Windows: Add PowerShell env support (#37951)

PowerShell requires explicit shell and env support in Spack.
This is due to the distinct differences in shell interactions between
cmd and pwsh. Add a doskey in pwsh piping 'spack' commands to a
powershell script similar to the sh function 'spack'. Add
support for PowerShell-specific shell interactions from Spack
(set/unset shell variables).

share/spack/setup-env.ps1

@@ -46,6 +46,10 @@ if ($null -eq $Env:EDITOR)
     $Env:EDITOR = "notepad"
 }
 
+# Set spack shell so we can detect powershell context
+$Env:SPACK_SHELL="pwsh"
+
+doskey /exename=powershell.exe spack=$Env:SPACK_ROOT\bin\spack.ps1 $args
 
 Write-Output "*****************************************************************"
 Write-Output "**************** Spack Package Manager **************************"

share/spack/spack-completion.bash

@@ -935,7 +935,7 @@ _spack_env() {
 _spack_env_activate() {
     if $list_options
     then
-        SPACK_COMPREPLY="-h --help --sh --csh --fish --bat -v --with-view -V --without-view -p --prompt --temp -d --dir"
+        SPACK_COMPREPLY="-h --help --sh --csh --fish --bat --pwsh -v --with-view -V --without-view -p --prompt --temp -d --dir"
     else
         _environments
     fi