feat: add prism sample.

demo/Ursa.PrismDialogDemo/MainWindow.axaml.cs

@@ -0,0 +1,21 @@
+using Avalonia.Controls;
+using Avalonia.Interactivity;
+using DryIoc;
+using Ursa.PrismExtension;
+
+namespace Ursa.PrismDialogDemo;
+
+public partial class MainWindow : Window
+{
+    private IUrsaOverlayDialogService _ursa;
+    public MainWindow(IUrsaOverlayDialogService ursa)
+    {
+        InitializeComponent();
+        _ursa = ursa;
+    }
+
+    private void Button_OnClick(object? sender, RoutedEventArgs e)
+    {
+        _ursa.ShowModal("Default", null, null, null);
+    }
+}