feat: add default template.

demo/Ursa.Demo/ViewModels/MessageBoxDemoViewModel.cs

@@ -0,0 +1,22 @@
+using System.Threading.Tasks;
+using System.Windows.Input;
+using CommunityToolkit.Mvvm.ComponentModel;
+using CommunityToolkit.Mvvm.Input;
+using Ursa.Controls;
+
+namespace Ursa.Demo.ViewModels;
+
+public class MessageBoxDemoViewModel: ObservableObject
+{
+    public ICommand DefaultMessageBoxCommand { get; set; }
+
+    public MessageBoxDemoViewModel()
+    {
+        DefaultMessageBoxCommand = new AsyncRelayCommand(OnDefaultMessageAsync);
+    }
+
+    private async Task OnDefaultMessageAsync()
+    {
+        await MessageBox.ShowAsync("Hello Message Box");
+    }
+}