CmdPal: Remove ShellViewModel's dependency on MainListPage (#40482)

targets #40479

Kinda mental that the viewmodel had this hard dependency.

So instead I added a service for providing the root page for the app.
Theoretically, you could add a different `IRootPageService`, and give
out
a different root page.


ref #40113

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

@@ -144,6 +144,7 @@ public partial class App : Application
         services.AddSingleton<IExtensionService, ExtensionService>();
         services.AddSingleton<TrayIconService>();
 
+        services.AddSingleton<IRootPageService, PowerToysRootPageService>();
         services.AddSingleton(new TelemetryForwarder());
 
         // ViewModels