Add scroll mouse manipulator

MaterialGraphProject/Assets/UnityShaderEditor/Editor/Drawing/Manipulators/Scrollable.cs

+using System;
+using UnityEngine.Experimental.UIElements;
+
+namespace UnityEditor.ShaderGraph.Drawing
+{
+    public class Scrollable : MouseManipulator
+    {
+        Action<float> m_Handler;
+
+        public Scrollable(Action<float> handler)
+        {
+            m_Handler = handler;
+            activators.Add(new ManipulatorActivationFilter()
+            {
+                button = MouseButton.LeftMouse
+            });
+        }
+
+        protected override void RegisterCallbacksOnTarget()
+        {
+            target.RegisterCallback(new EventCallback<WheelEvent>(OnMouseWheel), Capture.NoCapture);
+        }
+
+        protected override void UnregisterCallbacksFromTarget()
+        {
+            target.UnregisterCallback(new EventCallback<WheelEvent>(OnMouseWheel), Capture.NoCapture);
+        }
+
+        void OnMouseWheel(WheelEvent evt)
+        {
+            m_Handler(evt.delta.y);
+        }
+    }
+}

MaterialGraphProject/Assets/UnityShaderEditor/Editor/Drawing/Manipulators/Scrollable.cs.meta

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