CmdPal: Pull out VM bits from ShellPage.xaml.cs (#40479)

ref #40113

Moves a lot of the "model" logic out of `ShellPage.xaml.cs` into
`ShellViewModel`.

The LARGE majority of this code is copy-paste moving code. We're now
using a couple more messages to pass navigation between the VM and the
page. And a couple new messages for passing ETW events.

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

@@ -144,6 +144,8 @@ public partial class App : Application
         services.AddSingleton<IExtensionService, ExtensionService>();
         services.AddSingleton<TrayIconService>();
 
+        services.AddSingleton(new TelemetryForwarder());
+
         // ViewModels
         services.AddSingleton<ShellViewModel>();