feat: add new Toast.

demo/Ursa.Demo/Pages/ToastDemo.axaml

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+<UserControl x:Class="Ursa.Demo.Pages.ToastDemo"
+             xmlns="https://github.com/avaloniaui"
+             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+             xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
+             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
+             xmlns:vm="using:Ursa.Demo.ViewModels"
+             x:DataType="vm:ToastDemoViewModel"
+             d:DesignHeight="450"
+             d:DesignWidth="800"
+             mc:Ignorable="d">
+    <Design.DataContext>
+        <vm:ToastDemoViewModel />
+    </Design.DataContext>
+    <StackPanel>
+        <StackPanel Orientation="Horizontal" Spacing="20">
+            <Button Command="{Binding ShowNormal}" CommandParameter="{Binding $self.Content}" Content="Information" />
+            <Button Command="{Binding ShowNormal}" CommandParameter="{Binding $self.Content}" Content="Success" Classes="Success" />
+            <Button Command="{Binding ShowNormal}" CommandParameter="{Binding $self.Content}" Content="Warning" Classes="Warning" />
+            <Button Command="{Binding ShowNormal}" CommandParameter="{Binding $self.Content}" Content="Error" Classes="Danger" />
+        </StackPanel>
+        <StackPanel Orientation="Horizontal" Spacing="20">
+            <StackPanel.Styles>
+                <Style Selector="Button">
+                    <Setter Property="Theme" Value="{DynamicResource SolidButton}" />
+                </Style>
+            </StackPanel.Styles>
+            <Button Command="{Binding ShowLight}" CommandParameter="{Binding $self.Content}" Content="Information" />
+            <Button Command="{Binding ShowLight}" CommandParameter="{Binding $self.Content}" Content="Success" Classes="Success" />
+            <Button Command="{Binding ShowLight}" CommandParameter="{Binding $self.Content}" Content="Warning" Classes="Warning" />
+            <Button Command="{Binding ShowLight}" CommandParameter="{Binding $self.Content}" Content="Error" Classes="Danger" />
+        </StackPanel>
+    </StackPanel>
+</UserControl>

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

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+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 };
+    }
+}