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.Common/Services/IRootPageService.cs

@@ -1,37 +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 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);
-}