CmdPal: Did someone say grid pages? (#40832)

Closes #38269

Still working on this one, but essentially allows a list page to become
a grid page by specifying a `GridProperties` property like so:

```C#
public AllAppsPage()
{
    PlaceholderText = Resources.search_installed_apps_placeholder;
    GridProperties = new MediumGridLayout();
}
```

> This is a very early version and very subject to change. Much to
clean, but feedback & suggestions are welcome.

## Current preview

### SmallGridLayout

<img width="998" height="607" alt="image"
src="https://github.com/user-attachments/assets/ebdf11fd-6c86-4fc3-bf49-bcbb5d32caa4"
/>


### MediumGridLayout

<img width="976" height="586" alt="image"
src="https://github.com/user-attachments/assets/82daa2e9-548e-4864-8885-1c486ca9f891"
/>

### GalleryGridLayout

<img width="988" height="600" alt="image"
src="https://github.com/user-attachments/assets/23ca486a-35c7-467a-b200-4f6ee5f4a95c"
/>

---------

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

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

@@ -0,0 +1,32 @@
+// 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.
+
+namespace Microsoft.CommandPalette.Extensions.Toolkit;
+
+public partial class GalleryGridLayout : BaseObservable, IGalleryGridLayout
+{
+    public virtual bool ShowTitle
+    {
+        get => field;
+        set
+        {
+            field = value;
+            OnPropertyChanged(nameof(ShowTitle));
+        }
+    }
+
+    = true;
+
+    public virtual bool ShowSubtitle
+    {
+        get => field;
+        set
+        {
+            field = value;
+            OnPropertyChanged(nameof(ShowSubtitle));
+        }
+    }
+
+    = true;
+}

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

@@ -0,0 +1,20 @@
+// 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.
+
+namespace Microsoft.CommandPalette.Extensions.Toolkit;
+
+public partial class MediumGridLayout : BaseObservable, IMediumGridLayout
+{
+    public virtual bool ShowTitle
+    {
+        get => field;
+        set
+        {
+            field = value;
+            OnPropertyChanged(nameof(ShowTitle));
+        }
+    }
+
+    = true;
+}

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

@@ -0,0 +1,9 @@
+// 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.
+
+namespace Microsoft.CommandPalette.Extensions.Toolkit;
+
+public partial class SmallGridLayout : BaseObservable, ISmallGridLayout
+{
+}