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/ExtensionObjectViewModel.cs

@@ -0,0 +1,90 @@
+// 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 ManagedCommon;
+
+namespace Microsoft.CmdPal.Core.ViewModels;
+
+public abstract partial class ExtensionObjectViewModel : ObservableObject
+{
+    public WeakReference<IPageContext> PageContext { get; set; }
+
+    public ExtensionObjectViewModel(IPageContext? context)
+    {
+        var realContext = context ?? (this is IPageContext c ? c : throw new ArgumentException("You need to pass in an IErrorContext"));
+        PageContext = new(realContext);
+    }
+
+    public ExtensionObjectViewModel(WeakReference<IPageContext> context)
+    {
+        PageContext = context;
+    }
+
+    public async virtual Task InitializePropertiesAsync()
+    {
+        var t = new Task(() =>
+        {
+            SafeInitializePropertiesSynchronous();
+        });
+        t.Start();
+        await t;
+    }
+
+    public void SafeInitializePropertiesSynchronous()
+    {
+        try
+        {
+            InitializeProperties();
+        }
+        catch (Exception ex)
+        {
+            ShowException(ex);
+        }
+    }
+
+    public abstract void InitializeProperties();
+
+    protected void UpdateProperty(string propertyName)
+    {
+        DoOnUiThread(() => OnPropertyChanged(propertyName));
+    }
+
+    protected void ShowException(Exception ex, string? extensionHint = null)
+    {
+        if (PageContext.TryGetTarget(out var pageContext))
+        {
+            pageContext.ShowException(ex, extensionHint);
+        }
+    }
+
+    protected void DoOnUiThread(Action action)
+    {
+        if (PageContext.TryGetTarget(out var pageContext))
+        {
+            Task.Factory.StartNew(
+                action,
+                CancellationToken.None,
+                TaskCreationOptions.None,
+                pageContext.Scheduler);
+        }
+    }
+
+    protected virtual void UnsafeCleanup()
+    {
+        // base doesn't do anything, but sub-classes should override this.
+    }
+
+    public virtual void SafeCleanup()
+    {
+        try
+        {
+            UnsafeCleanup();
+        }
+        catch (Exception ex)
+        {
+            Logger.LogDebug(ex.ToString());
+        }
+    }
+}