feat: start/end effect.

src/Ursa/Controls/Timeline/Timeline.cs

@@ -83,6 +83,9 @@ public class Timeline: ItemsControl
         base.PrepareContainerForItemOverride(container, item, index);
         if (container is TimelineItem t)
         {
+            bool start = index == 0;
+            bool end = index == ItemCount - 1;
+            t.SetEnd(start, end);
             if (IconMemberBinding != null)
             {
                 t.Bind(TimelineItem.IconProperty, IconMemberBinding);

src/Ursa/Controls/Timeline/TimelineItem.cs

@@ -8,6 +8,7 @@ using Avalonia.Media;
 
 namespace Ursa.Controls;
 
+[PseudoClasses(":first", ":last")]
 public class TimelineItem: HeaderedContentControl
 {
     public static readonly StyledProperty<object?> IconProperty = AvaloniaProperty.Register<TimelineItem, object?>(
@@ -36,6 +37,10 @@ public class TimelineItem: HeaderedContentControl
         get => GetValue(TypeProperty);
         set => SetValue(TypeProperty, value);
     }
-    
-    
+
+    internal void SetEnd(bool start, bool end)
+    {
+        PseudoClasses.Set(":first", start);
+        PseudoClasses.Set(":last", end);
+    }
 }