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/IconDataViewModel.cs

@@ -0,0 +1,47 @@
+// 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 CommunityToolkit.Mvvm.ComponentModel;
+using Microsoft.CmdPal.Core.ViewModels.Models;
+using Microsoft.CommandPalette.Extensions;
+using Windows.Storage.Streams;
+
+namespace Microsoft.CmdPal.Core.ViewModels;
+
+public partial class IconDataViewModel : ObservableObject, IIconData
+{
+    private readonly ExtensionObject<IIconData> _model = new(null);
+
+    // If the extension previously gave us a Data, then died, the data will
+    // throw if we actually try to read it, but the pointer itself won't be
+    // null, so this is relatively safe.
+    public bool HasIcon => !string.IsNullOrEmpty(Icon) || Data.Unsafe != null;
+
+    // Locally cached properties from IIconData.
+    public string Icon { get; private set; } = string.Empty;
+
+    // Streams are not trivially copy-able, so we can't copy the data locally
+    // first. Hence why we're sticking this into an ExtensionObject
+    public ExtensionObject<IRandomAccessStreamReference> Data { get; private set; } = new(null);
+
+    IRandomAccessStreamReference? IIconData.Data => Data.Unsafe;
+
+    public IconDataViewModel(IIconData? icon)
+    {
+        _model = new(icon);
+    }
+
+    // Unsafe, needs to be called on BG thread
+    public void InitializeProperties()
+    {
+        var model = _model.Unsafe;
+        if (model == null)
+        {
+            return;
+        }
+
+        Icon = model.Icon;
+        Data = new(model.Data);
+    }
+}