[CmdPal] Add Sections and Separators for List Pages and Grid Pages (#43952)

<!-- Enter a brief description/summary of your PR here. What does it
fix/what does it change/how was it tested (even manually, if necessary)?
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## Summary of the Pull Request

This pull request adds sections and separators to ListPages and
GridPages

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## PR Checklist

- [x] Closes: #38267
<!-- - [ ] Closes: #yyy (add separate lines for additional resolved
issues) -->
- [ ] **Communication:** I've discussed this with core contributors
already. If the work hasn't been agreed, this work might be rejected
- [ ] **Tests:** Added/updated and all pass
- [ ] **Localization:** All end-user-facing strings can be localized
- [ ] **Dev docs:** Added/updated
- [ ] **New binaries:** Added on the required places
- [ ] [JSON for
signing](https://github.com/microsoft/PowerToys/blob/main/.pipelines/ESRPSigning_core.json)
for new binaries
- [ ] [WXS for
installer](https://github.com/microsoft/PowerToys/blob/main/installer/PowerToysSetup/Product.wxs)
for new binaries and localization folder
- [ ] [YML for CI
pipeline](https://github.com/microsoft/PowerToys/blob/main/.pipelines/ci/templates/build-powertoys-steps.yml)
for new test projects
- [ ] [YML for signed
pipeline](https://github.com/microsoft/PowerToys/blob/main/.pipelines/release.yml)
- [ ] **Documentation updated:** If checked, please file a pull request
on [our docs
repo](https://github.com/MicrosoftDocs/windows-uwp/tree/docs/hub/powertoys)
and link it here: #xxx

<!-- Provide a more detailed description of the PR, other things fixed,
or any additional comments/features here -->
## Detailed Description of the Pull Request / Additional comments

Since `CollectionViewSource` was causing performance issues and
@zadjii-msft asked for a new approach, I came up with this idea, heavily
inspired by how separators work on the `ContextMenu`, `FiltersDropDown`
and `Details`.

The way this is currently working is: Any ListItem where `Section` is
not null and `Command` is null, is considered a Separator.

On my tests, this seems to be working fine. Tried to make this work
without changes to the API, but I think this needs to be discussed.

### Some of the possible enhancements to existing extensions

### Search apps

<img width="792" height="523" alt="Screenshot 2025-11-27 173618"
src="https://github.com/user-attachments/assets/f9f9a64d-3ec1-4f7e-922b-997a3a4d074d"
/>

### Window Walker

<img width="785" height="518" alt="Screenshot 2025-11-27 173728"
src="https://github.com/user-attachments/assets/230f647d-210a-4b60-9068-c8fff890d2c9"
/>

### Winget

<img width="809" height="497" alt="Screenshot 2025-11-27 174006"
src="https://github.com/user-attachments/assets/547529c1-7600-4438-8c3e-e872e0327650"
/>

### Search files

<img width="819" height="536" alt="image"
src="https://github.com/user-attachments/assets/e86accc0-3f85-412d-8fb0-914a5479baff"
/>

### Grid Pages

<img width="804" height="964" alt="Screenshot 2025-11-27 174055"
src="https://github.com/user-attachments/assets/a3bba7db-95df-47ec-9cfb-f38775ab960e"
/>



<!-- Describe how you validated the behavior. Add automated tests
wherever possible, but list manual validation steps taken as well -->
## Validation Steps Performed

src/modules/cmdpal/extensionsdk/Microsoft.CommandPalette.Extensions.Toolkit/Section.cs

@@ -0,0 +1,39 @@
+// 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;
+
+namespace Microsoft.CommandPalette.Extensions.Toolkit;
+
+public sealed partial class Section : IEnumerable<IListItem>
+{
+    public IListItem[] Items { get; set; } = [];
+
+    public string SectionTitle { get; set; } = string.Empty;
+
+    private Separator CreateSectionListItem()
+    {
+        return new Separator(SectionTitle);
+    }
+
+    public Section(string sectionName, IListItem[] items)
+    {
+        SectionTitle = sectionName;
+        var listItems = items.ToList();
+
+        if (listItems.Count > 0)
+        {
+            listItems.Insert(0, CreateSectionListItem());
+            Items = [.. listItems];
+        }
+    }
+
+    public Section()
+    {
+    }
+
+    public IEnumerator<IListItem> GetEnumerator() => Items.ToList().GetEnumerator();
+
+    IEnumerator IEnumerable.GetEnumerator() => GetEnumerator();
+}

src/modules/cmdpal/extensionsdk/Microsoft.CommandPalette.Extensions.Toolkit/Separator.cs

@@ -4,6 +4,40 @@
 
 namespace Microsoft.CommandPalette.Extensions.Toolkit;
 
-public partial class Separator : ISeparatorContextItem, ISeparatorFilterItem
+public partial class Separator : IListItem, ISeparatorContextItem, ISeparatorFilterItem
 {
+    public Separator(string? title = "")
+        : base()
+    {
+        Section = title ?? string.Empty;
+        Command = null;
+    }
+
+    public IDetails? Details => null;
+
+    public string? Section { get; private set; }
+
+    public ITag[]? Tags => null;
+
+    public string? TextToSuggest => null;
+
+    public ICommand? Command { get; private set; }
+
+    public IIconInfo? Icon => null;
+
+    public IContextItem[]? MoreCommands => null;
+
+    public string? Subtitle => null;
+
+    public string? Title
+    {
+        get => Section;
+        set => Section = value;
+    }
+
+    public event Windows.Foundation.TypedEventHandler<object, IPropChangedEventArgs>? PropChanged
+    {
+        add { }
+        remove { }
+    }
 }