feat: add default template.

demo/Ursa.Demo/Pages/MessageBoxDemo.axaml

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+<UserControl 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"
+             mc:Ignorable="d" d:DesignWidth="800" d:DesignHeight="450"
+             xmlns:vm="using:Ursa.Demo.ViewModels"
+             x:DataType="vm:MessageBoxDemoViewModel"
+             x:CompileBindings="True"
+             x:Class="Ursa.Demo.Pages.MessageBoxDemo">
+    <StackPanel HorizontalAlignment="Left">
+        <Button Content="Default" Command="{Binding DefaultMessageBoxCommand}"></Button>
+    </StackPanel>
+</UserControl>

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

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+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();
+    }
+}