feat: add mvvm demo.

demo/Ursa.Demo/Views/MvvmSplashWindow.axaml

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+<u:SplashWindow 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:u="https://irihi.tech/ursa"
+        xmlns:viewModels="clr-namespace:Ursa.Demo.ViewModels"
+        xmlns:iri="https://irihi.tech/shared"
+        mc:Ignorable="d" 
+        Width="400" Height="400"
+        CountDown="{x:Null}"
+        x:DataType="viewModels:SplashViewModel"
+        x:Class="Ursa.Demo.Views.MvvmSplashWindow"
+        Title="MvvmSplashWindow">
+        <Grid
+                HorizontalAlignment="Center"
+                VerticalAlignment="Center"
+                ColumnDefinitions="Auto, Auto"
+                RowDefinitions="Auto, Auto, Auto">
+                <iri:IrihiLogo
+                        Grid.Row="0"
+                        Grid.Column="0"
+                        Width="64"
+                        Margin="0,0,16,0"
+                        Fill="{DynamicResource SemiGrey5}" />
+                <StackPanel Grid.Row="0" Grid.Column="1">
+                        <TextBlock
+                                Classes="H2"
+                                Text="铱泓科技"
+                                Theme="{DynamicResource TitleTextBlock}" />
+                        <TextBlock FontWeight="Bold" Text="IRIHI Technology" />
+                </StackPanel>
+                <ProgressBar Grid.Row="1" Grid.ColumnSpan="2" Value="{Binding Progress}" Margin="0 16 0 0"/>
+                <StackPanel
+                        Grid.Row="2"
+                        Grid.Column="0"
+                        Grid.ColumnSpan="2">
+                        <TextBlock
+                                Margin="0,12,0,0"
+                                HorizontalAlignment="Center"
+                                FontSize="14"
+                                Text="聚焦生产力的美学进化" />
+                        <TextBlock
+                                HorizontalAlignment="Center"
+                                FontSize="14"
+                                Text="Aesthetic Evolution of Productivity" />
+                </StackPanel>
+        </Grid>
+</u:SplashWindow>

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

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+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()
+        };
+    }
+}