feat: add button group.

demo/Ursa.Demo/ViewModels/ButtonGroupViewModel.cs

@@ -0,0 +1,11 @@
+using System.Collections.ObjectModel;
+
+namespace Ursa.Demo.ViewModels;
+
+public class ButtonGroupViewModel: ViewModelBase
+{
+    public ObservableCollection<string> Items { get; set; } = new ObservableCollection<string>()
+    {
+        "Ding", "Otter", "Husky", "Mr. 17", "Cass"
+    };
+}