Fix tasks never completing if cancelled quickly

We were exiting SynchronousTask.ExecuteSynchronously early if the
cancellation was already requested. This was not setting our tcs state
correctly, causing the caller to just fall off without warning.

Author: SeeminglyScience

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

@@ -35,7 +35,7 @@ protected SynchronousTask(
             CancellationToken cancellationToken)
         {
             Logger = logger;
-            _taskCompletionSource = new TaskCompletionSource<TResult>();
+            _taskCompletionSource = new TaskCompletionSource<TResult>(TaskCreationOptions.RunContinuationsAsynchronously);
             _taskRequesterCancellationToken = cancellationToken;
             _executionCanceled = false;
         }
@@ -76,6 +76,7 @@ public void ExecuteSynchronously(CancellationToken executorCancellationToken)
         {
             if (IsCanceled)
             {
+                SetCanceled();
                 return;
             }