fix(complete): Fix PowerShell dynamic completion

[jennings]

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 on Windows. To avoid syntax errors, define the argument to Invoke-Expression as a here-string so the quotes don't create a syntax error.

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

[jennings]

I admit I haven't yet been able to test this by actually compiling anything (I am pretty new to Rust). I modified the script in my PowerShell profile to match the change in this PR:

Register-ArgumentCompleter -Native -CommandName jj -ScriptBlock {
    param($wordToComplete, $commandAst, $cursorPosition)

    $prev = $env:COMPLETE
    $env:COMPLETE = "powershell"
    $results = Invoke-Expression @"
& "C:\\Users\\StephenJennings\\.cargo\\bin\\jj.exe" -- $commandAst
"@
    if ($null -eq $prev) {
        Remove-Item Env:\COMPLETE
    } else {
        $env:COMPLETE = $prev
    }
    $results | ForEach-Object {
        $split = $_.Split("`t");
        $cmd = $split[0];

        if ($split.Length -eq 2) {
            $help = $split[1];
        }
        else {
            $help = $split[0];
        }

        [System.Management.Automation.CompletionResult]::new($cmd, $cmd, 'ParameterValue', $help)
    }
};

[epage]

Thanks!