fix(complete): Update PowerShell dynamic completion script

PowerShell does not support inline syntax for assigning environment
variables, so we must instead set the value before running the completer
and restore it after it exits.

The completer will often, if not always, be surrounded by double quotes.
To avoid syntax errors, define the argument to Invoke-Expression as a
here-string so the quotes are part of the expression.

Updates the instructions for adding the argument completer to the
profile. Piping a native command to Invoke-Expression invokes each line
separately. Adding `Out-String` to the pipeline ensures that
Invoke-Expression receives the whole script as a single value from the
pipeline.

Fixes: #5847

clap_complete/src/env/mod.rs

@@ -52,7 +52,9 @@
 //!
 //! Powershell
 //! ```powershell
-//! echo "COMPLETE=powershell your_program | Invoke-Expression" >> $PROFILE
+//! $env:COMPLETE = "powershell"
+//! echo "your_program | Out-String | Invoke-Expression" >> $PROFILE
+//! Remove-Item Env:\COMPLETE
 //! ```
 //!
 //! Zsh

clap_complete/src/env/shells.rs

@@ -274,13 +274,25 @@ impl EnvCompleter for Powershell {
         let completer =
             shlex::try_quote(completer).unwrap_or(std::borrow::Cow::Borrowed(completer));
 
+        // `completer` may or may not be surrounded by double quotes, enclosing
+        // the expression in a here-string ensures the whole thing is
+        // interpreted as the first argument to the call operator
         writeln!(
             buf,
             r#"
 Register-ArgumentCompleter -Native -CommandName {bin} -ScriptBlock {{
     param($wordToComplete, $commandAst, $cursorPosition)
 
-    $results = Invoke-Expression "{var}=powershell &{completer} -- $($commandAst.ToString())";
+    $prev = $env:{var}
+    $env:{var} = "powershell"
+    $results = Invoke-Expression @"
+& {completer} -- $commandAst
+"@
+    if ($null -eq $prev) {{
+        Remove-Item Env:\{var}
+    }} else {{
+        $env:{var} = $prev
+    }}
     $results | ForEach-Object {{
         $split = $_.Split("`t");
         $cmd = $split[0];