bpo-32718: Make Activate.ps1 for venv cross-platform and available on all platforms (GH-9321)

PowerShell Core 6.1 is the cross-platform port of Windows PowerShell. This change updates Activate.ps1 to not make Windows assumptions as well as installing it into the bin/Scripts directory on all operating systems.

Requires PowerShell Core 6.1 for proper readline support once the shell has been activated for the virtual environment.

Lib/venv/scripts/common/Activate.ps1

@@ -0,0 +1,56 @@
+function Script:add-bin([string]$envPath) {
+    $binPath = Join-Path -Path $env:VIRTUAL_ENV -ChildPath '__VENV_BIN_NAME__'
+    return ($binPath, $envPath) -join [IO.Path]::PathSeparator
+}
+
+function global:deactivate ([switch]$NonDestructive) {
+    # Revert to original values
+    if (Test-Path function:_OLD_VIRTUAL_PROMPT) {
+        copy-item function:_OLD_VIRTUAL_PROMPT function:prompt
+        remove-item function:_OLD_VIRTUAL_PROMPT
+    }
+
+    if (Test-Path env:_OLD_VIRTUAL_PYTHONHOME) {
+        copy-item env:_OLD_VIRTUAL_PYTHONHOME env:PYTHONHOME
+        remove-item env:_OLD_VIRTUAL_PYTHONHOME
+    }
+
+    if (Test-Path env:_OLD_VIRTUAL_PATH) {
+        copy-item env:_OLD_VIRTUAL_PATH env:PATH
+        remove-item env:_OLD_VIRTUAL_PATH
+    }
+
+    if (Test-Path env:VIRTUAL_ENV) {
+        remove-item env:VIRTUAL_ENV
+    }
+
+    if (!$NonDestructive) {
+        # Self destruct!
+        remove-item function:deactivate
+    }
+}
+
+deactivate -nondestructive
+
+$env:VIRTUAL_ENV="__VENV_DIR__"
+
+if (! $env:VIRTUAL_ENV_DISABLE_PROMPT) {
+    # Set the prompt to include the env name
+    # Make sure _OLD_VIRTUAL_PROMPT is global
+    function global:_OLD_VIRTUAL_PROMPT {""}
+    copy-item function:prompt function:_OLD_VIRTUAL_PROMPT
+    function global:prompt {
+        Write-Host -NoNewline -ForegroundColor Green '__VENV_PROMPT__'
+        _OLD_VIRTUAL_PROMPT
+    }
+}
+
+# Clear PYTHONHOME
+if (Test-Path env:PYTHONHOME) {
+    copy-item env:PYTHONHOME env:_OLD_VIRTUAL_PYTHONHOME
+    remove-item env:PYTHONHOME
+}
+
+# Add the venv to the PATH
+copy-item env:PATH env:_OLD_VIRTUAL_PATH
+$env:PATH = add-bin $env:PATH