CmdPal: Add a viewmodel factory for pages (#40504)

_targets #40482_
ref #40113

A smaller refactor, to be sure.

This just moves the instantiation of PageViewModel objects out of the
ShellViewModel, and into its own class.

The idea being that other page types could be added, just by extending
that factory (or implementing your own), and then also handling those
new VMs in your ShellPage.xaml.cs equivalent.

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

@@ -149,6 +149,7 @@ public partial class App : Application
 
         // ViewModels
         services.AddSingleton<ShellViewModel>();
+        services.AddSingleton<IPageViewModelFactoryService, PageViewModelFactory>();
 
         return services.BuildServiceProvider();
     }