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.Common/Services/IExtensionService.cs

@@ -19,13 +19,13 @@ public interface IExtensionService
 
     Task SignalStopExtensionsAsync();
 
-    public event TypedEventHandler<IExtensionService, IEnumerable<IExtensionWrapper>>? OnExtensionAdded;
+    event TypedEventHandler<IExtensionService, IEnumerable<IExtensionWrapper>>? OnExtensionAdded;
 
-    public event TypedEventHandler<IExtensionService, IEnumerable<IExtensionWrapper>>? OnExtensionRemoved;
+    event TypedEventHandler<IExtensionService, IEnumerable<IExtensionWrapper>>? OnExtensionRemoved;
 
-    public void EnableExtension(string extensionUniqueId);
+    void EnableExtension(string extensionUniqueId);
 
-    public void DisableExtension(string extensionUniqueId);
+    void DisableExtension(string extensionUniqueId);
 
     ///// <summary>
     ///// Gets a boolean indicating whether the extension was disabled due to the corresponding Windows optional feature

src/modules/cmdpal/Microsoft.CmdPal.Common/Services/IRootPageService.cs

@@ -0,0 +1,37 @@
+// 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 System.Threading.Tasks;
+
+namespace Microsoft.CmdPal.Common.Services;
+
+public interface IRootPageService
+{
+    /// <summary>
+    /// Gets the root page of the command palette. Return any IPage implementation that
+    /// represents the root view of this instance of the command palette.
+    /// </summary>
+    Microsoft.CommandPalette.Extensions.IPage GetRootPage();
+
+    /// <summary>
+    /// Pre-loads any necessary data or state before the root page is loaded.
+    /// This will be awaited before the root page and the user can do anything,
+    /// so ideally it should be quick and not block the UI thread for long.
+    /// </summary>
+    Task PreLoadAsync();
+
+    /// <summary>
+    /// Do any loading work that can be done after the root page is loaded and
+    /// displayed to the user.
+    /// This is run asynchronously, on a background thread.
+    /// </summary>
+    Task PostLoadRootPageAsync();
+
+    /// <summary>
+    /// Called when a top-level command is performed. The context is the
+    /// sender context for the invoked command. This is typically the IListItem
+    /// or ICommandContextItem that was used to invoke the command.
+    /// </summary>
+    void OnPerformTopLevelCommand(object? context);
+}