Fixed Namespace usage and compilation errors

Editor/SceneViewToolbar/SceneViewToolbar.Overlays.cs

+#if UNITY_2021_2_OR_NEWER
 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 {
     public static partial class SceneViewToolbar
     {
-#if UNITY_2021_2_OR_NEWER
 
         [Overlay(typeof(SceneView), "Gameplay Ingredients", true)]
         public class IngredientsToolbarOverlay : ToolbarOverlay
                 }
             }
         }
-#endif
+#endif

Editor/SceneViewToolbar/SceneViewToolbar.cs

-using System.Collections;
-using UnityEditor.Overlays;
-using UnityEditor.Toolbars;
-using UnityEngine.UIElements;
 
 namespace GameplayIngredients.Editor
 {