[Revamp pipeline thread handling] Re-enable PS event handling

[SydneyhSmith]

Currently, PSES manages its events through PSRL's PS event handling functionality, which is here.

When PSRL idles, we run PowerShell events. And in fact today's PSES adds its own events to the event handler -- it's where the extension runs all its PowerShell executions for services. Which leads to issues like PowerShell/vscode-powershell#2448.

With the async pipeline thread consumer, we are now in control of all of this, but we lose PSRL's handling of it.

We either need to:

• Work out a way to get PSRL to handle it again, or
• Handle it ourselves

We have to remember that events can only be processed in a nested PowerShell.

The assumption currently is that we'll handle things ourselves and do it here:

PowerShellEditorServices/src/PowerShellEditorServices/Services/PowerShell/Execution/PipelineThreadExecutor.cs

Line 228 in a0a8df1

// TODO: Run nested pipeline here for engine event handling

[rjmholt]

To solve in #1459

[rjmholt]

Fixed in #1581. See #1591