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/Helpers/TelemetryForwarder.cs

@@ -0,0 +1,40 @@
+// Copyright (c) Microsoft Corporation
+// The Microsoft Corporation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using CommunityToolkit.Mvvm.Messaging;
+using Microsoft.CmdPal.UI.Events;
+using Microsoft.CmdPal.UI.ViewModels.Messages;
+using Microsoft.PowerToys.Telemetry;
+
+namespace Microsoft.CmdPal.UI;
+
+/// <summary>
+/// TelemetryForwarder is responsible for forwarding telemetry events from the
+/// command palette core to PowerToys Telemetry.
+/// This allows us to emit telemetry events as messages from the core,
+/// and then handle them by logging to our PT telemetry provider.
+///
+/// We may in the future want to replace this with a more generic "ITelemetryService"
+/// or something similar, but this works for now.
+/// </summary>
+internal sealed class TelemetryForwarder :
+    IRecipient<BeginInvokeMessage>,
+    IRecipient<CmdPalInvokeResultMessage>
+{
+    public TelemetryForwarder()
+    {
+        WeakReferenceMessenger.Default.Register<BeginInvokeMessage>(this);
+        WeakReferenceMessenger.Default.Register<CmdPalInvokeResultMessage>(this);
+    }
+
+    public void Receive(CmdPalInvokeResultMessage message)
+    {
+        PowerToysTelemetry.Log.WriteEvent(new CmdPalInvokeResult(message.Kind));
+    }
+
+    public void Receive(BeginInvokeMessage message)
+    {
+        PowerToysTelemetry.Log.WriteEvent(new BeginInvoke());
+    }
+}