feat: initialize marquee, add demo.

demo/Ursa.Demo/Pages/MarqueeDemo.axaml

@@ -0,0 +1,20 @@
+<UserControl
+    x:Class="Ursa.Demo.Pages.MarqueeDemo"
+    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"
+    d:DesignHeight="450"
+    d:DesignWidth="800"
+    mc:Ignorable="d">
+    <StackPanel>
+        <u:EnumSelector Name="direction" EnumType="{x:Type u:Direction}" />
+        <u:Marquee
+            Background="Red"
+            Height="100" 
+            Direction="{Binding #direction.Value}" >
+            <TextBlock Text="Hello World" VerticalAlignment="Center"/>
+        </u:Marquee>
+    </StackPanel>
+</UserControl>

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

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+using Avalonia;
+using Avalonia.Controls;
+using Avalonia.Markup.Xaml;
+
+namespace Ursa.Demo.Pages;
+
+public partial class MarqueeDemo : UserControl
+{
+    public MarqueeDemo()
+    {
+        InitializeComponent();
+    }
+}