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.Core.ViewModels/DetailsElementViewModel.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.CmdPal.Core.ViewModels.Models;
+using Microsoft.CommandPalette.Extensions;
+
+namespace Microsoft.CmdPal.Core.ViewModels;
+
+public abstract partial class DetailsElementViewModel(IDetailsElement _detailsElement, WeakReference<IPageContext> context) : ExtensionObjectViewModel(context)
+{
+    private readonly ExtensionObject<IDetailsElement> _model = new(_detailsElement);
+
+    public string Key { get; private set; } = string.Empty;
+
+    public override void InitializeProperties()
+    {
+        var model = _model.Unsafe;
+        if (model == null)
+        {
+            return;
+        }
+
+        Key = model.Key ?? string.Empty;
+        UpdateProperty(nameof(Key));
+    }
+}