CmdPal: Filters for DynamicListPage? Yes, please. (#40783)

Closes: #40382

## To-do list

- [x] Add support for "single-select" filters to DynamicListPage
- [x] Filters can contain icons
- [x] Filter list can contain separators
- [x] Update Windows Services built-in extension to support filtering by
all, started, stopped, and pending services
- [x] Update SampleExtension dynamic list sample to filter.

## Example of filters in use

```C#
internal sealed partial class ServicesListPage : DynamicListPage
{
    public ServicesListPage()
    {
        Icon = Icons.ServicesIcon;
        Name = "Windows Services";

        var filters = new ServiceFilters();
        filters.PropChanged += Filters_PropChanged;
        Filters = filters;
    }

    private void Filters_PropChanged(object sender, IPropChangedEventArgs args) => RaiseItemsChanged();

    public override void UpdateSearchText(string oldSearch, string newSearch) => RaiseItemsChanged();

    public override IListItem[] GetItems()
    {
       // ServiceHelper.Search knows how to filter based on the CurrentFilterIds provided
        var items = ServiceHelper.Search(SearchText, Filters.CurrentFilterIds).ToArray();

        return items;
    }
}

public partial class ServiceFilters : Filters
{
    public ServiceFilters()
    {
        // This would be a default selection. Not providing this will cause the filter
        // control to display the "Filter" placeholder text.
        CurrentFilterIds = ["all"];
    }

    public override IFilterItem[] GetFilters()
    {
        return [
            new Filter() { Id = "all", Name = "All Services" },
            new Separator(),
            new Filter() { Id = "running", Name = "Running", Icon = Icons.GreenCircleIcon },
            new Filter() { Id = "stopped", Name = "Stopped", Icon = Icons.RedCircleIcon },
            new Filter() { Id = "paused", Name = "Paused", Icon = Icons.PauseIcon },
        ];
    }
}
```

## Current example of behavior


https://github.com/user-attachments/assets/2e325763-ad3a-4445-bbe2-a840df08d0b3

---------

Co-authored-by: Mike Griese <migrie@microsoft.com>

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

@@ -2,6 +2,7 @@
 // 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;
 using System.Linq;
 using System.Runtime.InteropServices.WindowsRuntime;
 using Microsoft.CommandPalette.Extensions;
@@ -16,9 +17,14 @@ internal sealed partial class SampleDynamicListPage : DynamicListPage
         Icon = new IconInfo(string.Empty);
         Name = "Dynamic List";
         IsLoading = true;
+        var filters = new SampleFilters();
+        filters.PropChanged += Filters_PropChanged;
+        Filters = filters;
     }
 
-    public override void UpdateSearchText(string oldSearch, string newSearch) => RaiseItemsChanged(newSearch.Length);
+    private void Filters_PropChanged(object sender, IPropChangedEventArgs args) => RaiseItemsChanged();
+
+    public override void UpdateSearchText(string oldSearch, string newSearch) => RaiseItemsChanged();
 
     public override IListItem[] GetItems()
     {
@@ -28,6 +34,23 @@ internal sealed partial class SampleDynamicListPage : DynamicListPage
             items = [new ListItem(new NoOpCommand()) { Title = "Start typing in the search box" }];
         }
 
+        if (!string.IsNullOrEmpty(Filters.CurrentFilterId))
+        {
+            switch (Filters.CurrentFilterId)
+            {
+                case "mod2":
+                    items = items.Where((item, index) => (index + 1) % 2 == 0).ToArray();
+                    break;
+                case "mod3":
+                    items = items.Where((item, index) => (index + 1) % 3 == 0).ToArray();
+                    break;
+                case "all":
+                default:
+                    // No filtering
+                    break;
+            }
+        }
+
         if (items.Length > 0)
         {
             items[0].Subtitle = "Notice how the number of items changes for this page when you type in the filter box";
@@ -36,3 +59,18 @@ internal sealed partial class SampleDynamicListPage : DynamicListPage
         return items;
     }
 }
+
+#pragma warning disable SA1402 // File may only contain a single type
+public partial class SampleFilters : Filters
+#pragma warning restore SA1402 // File may only contain a single type
+{
+    public override IFilterItem[] GetFilters()
+    {
+        return
+        [
+            new Filter() { Id = "all", Name = "All" },
+            new Filter() { Id = "mod2", Name = "Every 2nd", Icon = new IconInfo("2") },
+            new Filter() { Id = "mod3", Name = "Every 3rd", Icon = new IconInfo("3") },
+        ];
+    }
+}

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

@@ -81,7 +81,7 @@ internal sealed partial class SampleListPage : ListPage
                         Title = "I'm a second command",
                         RequestedShortcut = KeyChordHelpers.FromModifiers(ctrl: true, vkey: VirtualKey.Number1),
                     },
-                    new SeparatorContextItem(),
+                    new Separator(),
                     new CommandContextItem(
                         new ToastCommand("Third command invoked", MessageState.Error) { Name = "Do 3", Icon = new IconInfo("\uF148") }) // dial 3
                     {