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.ViewModels/ContextMenuStackViewModel.cs

@@ -0,0 +1,82 @@
+// 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 CommunityToolkit.Mvvm.ComponentModel;
+using Microsoft.CmdPal.UI.ViewModels.Messages;
+using Microsoft.CommandPalette.Extensions.Toolkit;
+using Windows.System;
+
+namespace Microsoft.CmdPal.UI.ViewModels;
+
+public partial class ContextMenuStackViewModel : ObservableObject
+{
+    [ObservableProperty]
+    public partial ObservableCollection<CommandContextItemViewModel> FilteredItems { get; set; }
+
+    private readonly IContextMenuContext _context;
+    private string _lastSearchText = string.Empty;
+
+    // private Dictionary<KeyChord, CommandContextItemViewModel>? _contextKeybindings;
+    public ContextMenuStackViewModel(IContextMenuContext context)
+    {
+        _context = context;
+        FilteredItems = [.. context.AllCommands];
+    }
+
+    public void SetSearchText(string searchText)
+    {
+        if (searchText == _lastSearchText)
+        {
+            return;
+        }
+
+        _lastSearchText = searchText;
+
+        var commands = _context.AllCommands.Where(c => c.ShouldBeVisible);
+        if (string.IsNullOrEmpty(searchText))
+        {
+            ListHelpers.InPlaceUpdateList(FilteredItems, commands);
+            return;
+        }
+
+        var newResults = ListHelpers.FilterList<CommandContextItemViewModel>(commands, searchText, ScoreContextCommand);
+        ListHelpers.InPlaceUpdateList(FilteredItems, newResults);
+    }
+
+    private static int ScoreContextCommand(string query, CommandContextItemViewModel item)
+    {
+        if (string.IsNullOrEmpty(query) || string.IsNullOrWhiteSpace(query))
+        {
+            return 1;
+        }
+
+        if (string.IsNullOrEmpty(item.Title))
+        {
+            return 0;
+        }
+
+        var nameMatch = StringMatcher.FuzzySearch(query, item.Title);
+
+        var descriptionMatch = StringMatcher.FuzzySearch(query, item.Subtitle);
+
+        return new[] { nameMatch.Score, (descriptionMatch.Score - 4) / 2, 0 }.Max();
+    }
+
+    public CommandContextItemViewModel? CheckKeybinding(bool ctrl, bool alt, bool shift, bool win, VirtualKey key)
+    {
+        var keybindings = _context.Keybindings();
+        if (keybindings != null)
+        {
+            // Does the pressed key match any of the keybindings?
+            var pressedKeyChord = KeyChordHelpers.FromModifiers(ctrl, alt, shift, win, key, 0);
+            if (keybindings.TryGetValue(pressedKeyChord, out var item))
+            {
+                return item;
+            }
+        }
+
+        return null;
+    }
+}