Add private contract delegate for PSES to handle idle on readline

Author: rjmholt

PSReadLine.build.ps1

@@ -144,6 +144,10 @@ task LayoutModule BuildPolyfiller, BuildMainModule, {
     Copy-Item $binPath/Microsoft.PowerShell.PSReadLine2.dll $targetDir
     Copy-Item $binPath/Microsoft.PowerShell.Pager.dll $targetDir
 
+    if ($Configuration -eq 'Debug') {
+      Copy-Item $binPath/*.pdb $targetDir
+    }
+
     if (Test-Path $binPath/System.Runtime.InteropServices.RuntimeInformation.dll) {
         Copy-Item $binPath/System.Runtime.InteropServices.RuntimeInformation.dll $targetDir
     } else {

PSReadLine/ReadLine.cs

@@ -44,6 +44,8 @@ public partial class PSConsoleReadLine : IPSConsoleReadLineMockableMethods
 
         private static readonly CancellationToken _defaultCancellationToken = new CancellationTokenSource().Token;
 
+        private static Action _handleIdleOverride;
+
         private bool _delayedOneTimeInitCompleted;
 
         private IPSConsoleReadLineMockableMethods _mockableMethods;
@@ -201,6 +203,12 @@ internal static PSKeyInfo ReadKey()
                     if (handleId != WaitHandle.WaitTimeout && handleId != EventProcessingRequested)
                         break;
 
+                    if (_handleIdleOverride != null)
+                    {
+                        _handleIdleOverride();
+                        continue;
+                    }
+
                     // If we timed out, check for event subscribers (which is just
                     // a hint that there might be an event waiting to be processed.)
                     var eventSubscribers = _singleton._engineIntrinsics?.Events.Subscribers;