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/extensionsdk/Microsoft.CommandPalette.Extensions.Toolkit/ListItem.cs

@@ -61,4 +61,9 @@ public partial class ListItem : CommandItem, IListItem
         : base(command)
     {
     }
+
+    public ListItem()
+        : base()
+    {
+    }
 }