feat: add default template.

demo/Ursa.Demo/Pages/MessageBoxDemo.axaml.cs

@@ -0,0 +1,15 @@
+using Avalonia;
+using Avalonia.Controls;
+using Avalonia.Markup.Xaml;
+using Ursa.Demo.ViewModels;
+
+namespace Ursa.Demo.Pages;
+
+public partial class MessageBoxDemo : UserControl
+{
+    public MessageBoxDemo()
+    {
+        InitializeComponent();
+        this.DataContext = new MessageBoxDemoViewModel();
+    }
+}