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/IconInfoViewModel.cs

@@ -0,0 +1,57 @@
+// 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;
+
+namespace Microsoft.CmdPal.Core.ViewModels;
+
+public partial class IconInfoViewModel : ObservableObject, IIconInfo
+{
+    private readonly ExtensionObject<IIconInfo> _model = new(null);
+
+    // These are properties that are "observable" from the extension object
+    // itself, in the sense that they get raised by PropChanged events from the
+    // extension. However, we don't want to actually make them
+    // [ObservableProperty]s, because PropChanged comes in off the UI thread,
+    // and ObservableProperty is not smart enough to raise the PropertyChanged
+    // on the UI thread.
+    public IconDataViewModel Light { get; private set; }
+
+    public IconDataViewModel Dark { get; private set; }
+
+    public IconDataViewModel IconForTheme(bool light) => Light = light ? Light : Dark;
+
+    public bool HasIcon(bool light) => IconForTheme(light).HasIcon;
+
+    public bool IsSet => _model.Unsafe != null;
+
+    IIconData? IIconInfo.Dark => Dark;
+
+    IIconData? IIconInfo.Light => Light;
+
+    public IconInfoViewModel(IIconInfo? icon)
+    {
+        _model = new(icon);
+        Light = new(null);
+        Dark = new(null);
+    }
+
+    // Unsafe, needs to be called on BG thread
+    public void InitializeProperties()
+    {
+        var model = _model.Unsafe;
+        if (model == null)
+        {
+            return;
+        }
+
+        Light = new(model.Light);
+        Light.InitializeProperties();
+
+        Dark = new(model.Dark);
+        Dark.InitializeProperties();
+    }
+}