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.ViewModels/PageViewModelFactory.cs

@@ -0,0 +1,27 @@
+// 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 Microsoft.CommandPalette.Extensions;
+
+namespace Microsoft.CmdPal.UI.ViewModels;
+
+public class PageViewModelFactory : IPageViewModelFactoryService
+{
+    private readonly TaskScheduler _scheduler;
+
+    public PageViewModelFactory(TaskScheduler scheduler)
+    {
+        _scheduler = scheduler;
+    }
+
+    public PageViewModel? TryCreatePageViewModel(IPage page, bool nested, CommandPaletteHost host)
+    {
+        return page switch
+        {
+            IListPage listPage => new ListViewModel(listPage, _scheduler, host) { IsNested = nested },
+            IContentPage contentPage => new ContentPageViewModel(contentPage, _scheduler, host),
+            _ => null,
+        };
+    }
+}