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/Microsoft.CmdPal.UI/Pages/ShellPage.xaml.cs

@@ -187,8 +187,6 @@ public sealed partial class ShellPage : Microsoft.UI.Xaml.Controls.Page,
 
                     WeakReferenceMessenger.Default.Send<UpdateCommandBarMessage>(new(null));
 
-                    WeakReferenceMessenger.Default.Send<UpdateItemKeybindingsMessage>(new(null));
-
                     var isMainPage = command is MainListPage;
 
                     // Construct our ViewModel of the appropriate type and pass it the UI Thread context.