Add support for filterable, nested context menus (#38776)

_targets #38573_

At first I just wanted to add support for nested context menus.

But then I also had to add a search box, so the focus wouldn't get weird.

End result:

![nested-menus-001](https://github.com/user-attachments/assets/4e8f1ec8-4b09-4095-9b81-caf7abde8aea)

This gets rid of the need to have the search box and the command bar both track item keybindings - now it's just in the command bar.

Closes #38299
Closes #38442

src/modules/cmdpal/ext/SamplePagesExtension/Pages/ToastCommand.cs

@@ -0,0 +1,23 @@
+// 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.CommandPalette.Extensions;
+using Microsoft.CommandPalette.Extensions.Toolkit;
+
+namespace SamplePagesExtension;
+
+internal sealed partial class ToastCommand(string message, MessageState state = MessageState.Info) : InvokableCommand
+{
+    public override ICommandResult Invoke()
+    {
+        var t = new ToastStatusMessage(new StatusMessage()
+        {
+            Message = message,
+            State = state,
+        });
+        t.Show();
+
+        return CommandResult.KeepOpen();
+    }
+}