CmdPal: Use a factory for building the context menu VMs (#45572)

_targets #45566_

doesn't actually do anything, just moves around the instantiation of
command context item VMs.

This will let use add pin/unpin commands later

related to https://github.com/microsoft/PowerToys/issues/45191
related to https://github.com/microsoft/PowerToys/issues/45201

src/modules/cmdpal/Microsoft.CmdPal.UI/App.xaml.cs

@@ -220,6 +220,7 @@ public partial class App : Application, IDisposable
 
         // ViewModels
         services.AddSingleton<ShellViewModel>();
+        services.AddSingleton<IContextMenuFactory, CommandPaletteContextMenuFactory>();
         services.AddSingleton<IPageViewModelFactoryService, CommandPalettePageViewModelFactory>();
     }