chore: separated System.Windows.Interactivity code

src/Shared/System.Windows.Interactivity/Behavior`1.cs

@@ -0,0 +1,12 @@
+using System.Windows;
+
+namespace HandyControl.Interactivity;
+
+public abstract class Behavior<T> : Behavior where T : DependencyObject
+{
+    protected Behavior() : base(typeof(T))
+    {
+    }
+
+    protected new T AssociatedObject => (T) base.AssociatedObject;
+}