feat: add new Toast.

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

@@ -0,0 +1,25 @@
+using Avalonia;
+using Avalonia.Controls;
+using Ursa.Controls;
+using Ursa.Demo.ViewModels;
+
+namespace Ursa.Demo.Pages;
+
+public partial class ToastDemo : UserControl
+{
+    private ToastDemoViewModel _viewModel;
+
+    public ToastDemo()
+    {
+        InitializeComponent();
+        _viewModel = new ToastDemoViewModel();
+        DataContext = _viewModel;
+    }
+
+    protected override void OnAttachedToVisualTree(VisualTreeAttachmentEventArgs e)
+    {
+        base.OnAttachedToVisualTree(e);
+        var topLevel = TopLevel.GetTopLevel(this);
+        _viewModel.ToastManager = new WindowToastManager(topLevel) { MaxItems = 3 };
+    }
+}