feat: add loading.

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

@@ -0,0 +1,18 @@
+using Avalonia;
+using Avalonia.Controls;
+using Avalonia.Markup.Xaml;
+
+namespace Ursa.Demo.Pages;
+
+public partial class LoadingDemo : UserControl
+{
+    public LoadingDemo()
+    {
+        InitializeComponent();
+    }
+
+    private void InitializeComponent()
+    {
+        AvaloniaXamlLoader.Load(this);
+    }
+}