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/ProgressViewModel.cs

@@ -0,0 +1,70 @@
+// 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 partial class ProgressViewModel : ExtensionObjectViewModel
+{
+    public ExtensionObject<IProgressState> Model { get; }
+
+    public bool IsIndeterminate { get; private set; }
+
+    public uint ProgressPercent { get; private set; }
+
+    public ProgressViewModel(IProgressState progress, WeakReference<IPageContext> context)
+        : base(context)
+    {
+        Model = new(progress);
+    }
+
+    public override void InitializeProperties()
+    {
+        var model = Model.Unsafe;
+        if (model == null)
+        {
+            return; // throw?
+        }
+
+        IsIndeterminate = model.IsIndeterminate;
+        ProgressPercent = model.ProgressPercent;
+
+        model.PropChanged += Model_PropChanged;
+    }
+
+    private void Model_PropChanged(object sender, IPropChangedEventArgs args)
+    {
+        try
+        {
+            FetchProperty(args.PropertyName);
+        }
+        catch (Exception ex)
+        {
+            ShowException(ex);
+        }
+    }
+
+    protected virtual void FetchProperty(string propertyName)
+    {
+        var model = this.Model.Unsafe;
+        if (model == null)
+        {
+            return; // throw?
+        }
+
+        switch (propertyName)
+        {
+            case nameof(IsIndeterminate):
+                this.IsIndeterminate = model.IsIndeterminate;
+                break;
+            case nameof(ProgressPercent):
+                this.ProgressPercent = model.ProgressPercent;
+                break;
+        }
+
+        UpdateProperty(propertyName);
+    }
+}