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.ViewModels/LoadingPageViewModel.cs

@@ -1,17 +0,0 @@
-// 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 partial class LoadingPageViewModel : PageViewModel
-{
-    public LoadingPageViewModel(IPage? model, TaskScheduler scheduler)
-        : base(model, scheduler, CommandPaletteHost.Instance)
-    {
-        ModelIsLoading = true;
-        IsInitialized = false;
-    }
-}