WIP: Wrap ReadLine with lock and fallback to legacy when re-entering

[andyleejordan]

Just me trying things. So, weird thing, but in the repro for PowerShell/vscode-powershell#3751, while we're in the debugger, things can't print. Like if you run Get-Process at the debug prompt it shows nothing, and if you're debugging the pwsh process you see a million exceptions. This gets resolved when I set the host explicitly in the new PSInvocationSettings. When we either don't create the linked cancellation token source, or when we move its scope outside the function its currently created in, the repro issue goes away. The monitor logic I guess is useless 🥲

Play around with it @SeeminglyScience?

[SeeminglyScience]

Alright so think of a linked cancellation token source kinda like this:

static void MyMethod(CancellationToken callersToken)
{
    CancellationTokenSource myToken = new(millisecondsDelay: 5000);

    // Real method name is "Register", but bare with me
    callersToken.OnCancel(() => myToken.Cancel());
    OtherMethod(myToken.Token);
}

That's all linking does. The issue with the WIP here is instead of passing myToken.Token, you're passing callersToken. So if we call myToken.Cancel(), that's not going to do anything. That's also why it looks like it works: it's never cancelled.

I haven't played with it yet, but I would guess we can no longer cancel PSRL like this (e.g. F8/editor commands/continue). If we can still cancel PSRL, we should probably look at references for _taskRequesterCancellationToken to see if we ever even cancel it.

[andyleejordan]

Oops I meant to still send cancellationSource.Token so I clearly didn't test what I thought I was testing.