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/Commands/LogMessagesPage.cs

@@ -3,6 +3,7 @@
 // See the LICENSE file in the project root for more information.
 
 using System.Collections.Specialized;
+using Microsoft.CmdPal.Core.ViewModels;
 using Microsoft.CommandPalette.Extensions;
 using Microsoft.CommandPalette.Extensions.Toolkit;
 
@@ -10,7 +11,7 @@ namespace Microsoft.CmdPal.UI.ViewModels.Commands;
 
 public partial class LogMessagesPage : ListPage
 {
-    private readonly List<IListItem> _listItems = new();
+    private readonly List<IListItem> _listItems = [];
 
     public LogMessagesPage()
     {
@@ -31,7 +32,6 @@ public partial class LogMessagesPage : ListPage
                     var li = new ListItem(new NoOpCommand())
                     {
                         Title = logMessageViewModel.Message,
-                        Subtitle = logMessageViewModel.ExtensionPfn,
                     };
                     _listItems.Insert(0, li);
                 }