feat: add mvvm demo.

demo/Ursa.Demo/Views/MvvmSplashWindow.axaml.cs

@@ -0,0 +1,24 @@
+using System.Threading.Tasks;
+using Avalonia;
+using Avalonia.Controls;
+using Avalonia.Markup.Xaml;
+using Ursa.Controls;
+using Ursa.Demo.ViewModels;
+
+namespace Ursa.Demo.Views;
+
+public partial class MvvmSplashWindow : SplashWindow
+{
+    public MvvmSplashWindow()
+    {
+        InitializeComponent();
+    }
+
+    protected override async Task<Window> CreateNextWindow()
+    {
+        return new MainWindow()
+        {
+            DataContext = new MainViewViewModel()
+        };
+    }
+}