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/AppExtensionHost.cs

@@ -0,0 +1,168 @@
+// 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 System.Collections.ObjectModel;
+using System.Diagnostics;
+using ManagedCommon;
+using Microsoft.CommandPalette.Extensions;
+using Microsoft.CommandPalette.Extensions.Toolkit;
+using Windows.Foundation;
+
+namespace Microsoft.CmdPal.Core.ViewModels;
+
+public abstract partial class AppExtensionHost : IExtensionHost
+{
+    private static readonly GlobalLogPageContext _globalLogPageContext = new();
+
+    private static ulong _hostingHwnd;
+
+    public static ObservableCollection<LogMessageViewModel> LogMessages { get; } = [];
+
+    public ulong HostingHwnd => _hostingHwnd;
+
+    public string LanguageOverride => string.Empty;
+
+    public ObservableCollection<StatusMessageViewModel> StatusMessages { get; } = [];
+
+    public static void SetHostHwnd(ulong hostHwnd) => _hostingHwnd = hostHwnd;
+
+    public void DebugLog(string message)
+    {
+#if DEBUG
+        this.ProcessLogMessage(new LogMessage(message));
+#endif
+    }
+
+    public IAsyncAction HideStatus(IStatusMessage? message)
+    {
+        if (message == null)
+        {
+            return Task.CompletedTask.AsAsyncAction();
+        }
+
+        _ = Task.Run(() =>
+        {
+            ProcessHideStatusMessage(message);
+        });
+        return Task.CompletedTask.AsAsyncAction();
+    }
+
+    public void Log(string message)
+    {
+        this.ProcessLogMessage(new LogMessage(message));
+    }
+
+    public IAsyncAction LogMessage(ILogMessage? message)
+    {
+        if (message == null)
+        {
+            return Task.CompletedTask.AsAsyncAction();
+        }
+
+        Logger.LogDebug(message.Message);
+
+        _ = Task.Run(() =>
+        {
+            ProcessLogMessage(message);
+        });
+
+        // We can't just make a LogMessageViewModel : ExtensionObjectViewModel
+        // because we don't necessarily know the page context. Butts.
+        return Task.CompletedTask.AsAsyncAction();
+    }
+
+    public void ProcessHideStatusMessage(IStatusMessage message)
+    {
+        Task.Factory.StartNew(
+            () =>
+            {
+                try
+                {
+                    var vm = StatusMessages.Where(messageVM => messageVM.Model.Unsafe == message).FirstOrDefault();
+                    if (vm != null)
+                    {
+                        StatusMessages.Remove(vm);
+                    }
+                }
+                catch
+                {
+                }
+            },
+            CancellationToken.None,
+            TaskCreationOptions.None,
+            _globalLogPageContext.Scheduler);
+    }
+
+    public void ProcessLogMessage(ILogMessage message)
+    {
+        var vm = new LogMessageViewModel(message, _globalLogPageContext);
+        vm.SafeInitializePropertiesSynchronous();
+
+        Task.Factory.StartNew(
+            () =>
+            {
+                LogMessages.Add(vm);
+            },
+            CancellationToken.None,
+            TaskCreationOptions.None,
+            _globalLogPageContext.Scheduler);
+    }
+
+    public void ProcessStatusMessage(IStatusMessage message, StatusContext context)
+    {
+        // If this message is already in the list of messages, just bring it to the top
+        var oldVm = StatusMessages.Where(messageVM => messageVM.Model.Unsafe == message).FirstOrDefault();
+        if (oldVm != null)
+        {
+            Task.Factory.StartNew(
+                () =>
+                {
+                    StatusMessages.Remove(oldVm);
+                    StatusMessages.Add(oldVm);
+                },
+                CancellationToken.None,
+                TaskCreationOptions.None,
+                _globalLogPageContext.Scheduler);
+            return;
+        }
+
+        var vm = new StatusMessageViewModel(message, new(_globalLogPageContext));
+        vm.SafeInitializePropertiesSynchronous();
+
+        Task.Factory.StartNew(
+            () =>
+            {
+                StatusMessages.Add(vm);
+            },
+            CancellationToken.None,
+            TaskCreationOptions.None,
+            _globalLogPageContext.Scheduler);
+    }
+
+    public IAsyncAction ShowStatus(IStatusMessage? message, StatusContext context)
+    {
+        if (message == null)
+        {
+            return Task.CompletedTask.AsAsyncAction();
+        }
+
+        Debug.WriteLine(message.Message);
+
+        _ = Task.Run(() =>
+        {
+            ProcessStatusMessage(message, context);
+        });
+
+        return Task.CompletedTask.AsAsyncAction();
+    }
+
+    public abstract string? GetExtensionDisplayName();
+}
+
+public interface IAppHostService
+{
+    AppExtensionHost GetDefaultHost();
+
+    AppExtensionHost GetHostForCommand(object? context, AppExtensionHost? currentHost);
+}