feat: implement separator.

src/Ursa/Controls/Navigation/NavigationMenuSeparator.cs

@@ -1,6 +1,11 @@
+using Avalonia.Input;
+
 namespace Ursa.Controls;
 
 public class NavigationMenuSeparator: NavigationMenuItem
 {
-    
+    protected override void OnPointerPressed(PointerPressedEventArgs e)
+    {
+        e.Handled = true;
+    }
 }