Added Handler registrations in IServiceCollectionExtensions

Hyperbar.Windows/Mediators/KeyAcceleratorCommandHandler.cs

@@ -0,0 +1,11 @@
+namespace Hyperbar.Windows;
+
+public class KeyAcceleratorCommandHandler :
+    ICommandHandler<KeyAcceleratorCommand>
+{
+    public ValueTask<Unit> Handle(KeyAcceleratorCommand command,
+        CancellationToken cancellationToken)
+    {
+        throw new NotImplementedException();
+    }
+}