Create a Microsoft.CmdPal.Core.ViewModels project (#40560)

_targets #40504_ 

Major refactoring for #40113

This moves a large swath of the codebase to a `.Core` project. "Core"
doesn't have any explicit dependencies on "extensions", settings or the
current `MainListPage`. It's just a filterable list of stuff. This
should let us make this component a bit more reusable.

This is half of a PR. As I did this, I noticed a particular bit of code
for TopLevelVViewModels and CommandPaletteHost that was _very rough_.
Solving it in this PR would make "move everything to a new project" much
harder to review. So I'm submitting two PRs simultaneously, so we can
see the changes separately, then merge together.

src/modules/cmdpal/Microsoft.CmdPal.UI/PowerToysAppHostService.cs

@@ -0,0 +1,29 @@
+// 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.CmdPal.Core.ViewModels;
+using Microsoft.CmdPal.UI.ViewModels;
+
+// To learn more about WinUI, the WinUI project structure,
+// and more about our project templates, see: http://aka.ms/winui-project-info.
+namespace Microsoft.CmdPal.UI;
+
+internal sealed class PowerToysAppHostService : IAppHostService
+{
+    public AppExtensionHost GetDefaultHost()
+    {
+        return CommandPaletteHost.Instance;
+    }
+
+    public AppExtensionHost GetHostForCommand(object? context, AppExtensionHost? currentHost)
+    {
+        AppExtensionHost? topLevelHost = null;
+        if (context is TopLevelViewModel topLevelViewModel)
+        {
+            topLevelHost = topLevelViewModel.ExtensionHost;
+        }
+
+        return topLevelHost ?? currentHost ?? CommandPaletteHost.Instance;
+    }
+}