Use primary constructor for QuadtreeItem

Quadtree/QuadtreeItem.cs

@@ -1,22 +1,16 @@
 namespace SpatialCollections
 {
-    internal struct QuadtreeItem<T>
+    internal struct QuadtreeItem<T>(T entity, int next)
     {
         /// <summary>
         /// Index of the next item in the same leaf vertex.
         /// </summary>
-        public int Next;
+        public int Next = next;
 
-        public T Entity;
+        public T Entity = entity;
 
         public QuadtreeItem(T entity) : this(entity, -1)
         {
         }
-
-        public QuadtreeItem(T entity, int next)
-        {
-            Next = next;
-            Entity = entity;
-        }
     }
 }