Created a nested PowerShell for the top-level loop

[andyleejordan]

So that events registered with the OnIdle engine event handler will always work.

Resolves PowerShell/vscode-powershell#4048

I added the following code to my profile:

Register-EngineEvent -SourceIdentifier PowerShell.OnIdle -MaxTriggerCount 10 -Action {
  Write-Host "OnIdle"
}

and was able to repro the linked issue before this change, and cannot repro it afterwards.

There may be other ramifications that we're not seeing though, and I'd be curious about a regression test for this.

[SeeminglyScience]

LGTM! Nice approach ❤️ my take was going to be a little bit more hacky since it would happen earlier in start up. This is better

[andyleejordan]

The plus side of spending that feverish day looking at the Get-Variable code was that I got a better understanding of the runspace/nested/availability stuff. It's still pretty obtuse to me, but hey this works.