Don't use RunContinuationsAsynchronously for our TaskCompletionSource (#1838)

It's not a good sign that this slowed everything down, but at least it's
easy to revert. More investigation is required.

Author: SeeminglyScience

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

@@ -35,7 +35,7 @@ protected SynchronousTask(
             CancellationToken cancellationToken)
         {
             Logger = logger;
-            _taskCompletionSource = new TaskCompletionSource<TResult>(TaskCreationOptions.RunContinuationsAsynchronously);
+            _taskCompletionSource = new TaskCompletionSource<TResult>();
             _taskRequesterCancellationToken = cancellationToken;
             _executionCanceled = false;
         }