feat: WIP add a demo.

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

@@ -0,0 +1,13 @@
+using Avalonia;
+using Avalonia.Controls;
+using Avalonia.Markup.Xaml;
+
+namespace Ursa.Demo.Pages;
+
+public partial class MultiComboBoxDemo : UserControl
+{
+    public MultiComboBoxDemo()
+    {
+        InitializeComponent();
+    }
+}