feat: add mvvm demo.

demo/Ursa.Demo/ViewModels/SplashViewModel.cs

@@ -0,0 +1,38 @@
+using System;
+using Avalonia.Threading;
+using CommunityToolkit.Mvvm.ComponentModel;
+using Irihi.Avalonia.Shared.Contracts;
+
+namespace Ursa.Demo.ViewModels;
+
+public partial class SplashViewModel: ObservableObject, IDialogContext
+{
+    [ObservableProperty] private double _progress;
+    private Random _r = new();
+
+    public SplashViewModel()
+    {
+        DispatcherTimer.Run(OnUpdate, TimeSpan.FromMilliseconds(200), DispatcherPriority.Default);
+    }
+
+    private bool OnUpdate()
+    {
+        Progress += 10 * _r.NextDouble();
+        if (Progress <= 100)
+        {
+            return true;
+        }
+        else
+        {
+            RequestClose?.Invoke(this, null);
+            return false;
+        }
+    }
+    
+    public void Close()
+    {
+        RequestClose?.Invoke(this, null);
+    }
+
+    public event EventHandler<object?>? RequestClose;
+}