add unity behavior tree package & examples

add start of own implementation of a behavior tree

AI/Assets/Scripts/BehaviorTree/Node.cs

@@ -0,0 +1,31 @@
+using System.Collections.Generic;
+
+namespace DefaultNamespace.BehaviorTree
+{
+    public abstract class Node
+    {
+        protected List<Node> _nodes = new();
+
+        public virtual void Start()
+        {
+            
+        }
+
+        public virtual Status Update()
+        {
+            return Status.Success;
+        }
+
+        public virtual void End()
+        {
+            
+        }
+    }
+
+    public enum Status
+    {
+        Success,
+        Failure,
+        Running,
+    }
+}