add unity behavior tree package & examples

add start of own implementation of a behavior tree

AI/Assets/Scripts/BehaviorTree/Tree.cs

@@ -0,0 +1,28 @@
+using System;
+using UnityEngine;
+
+namespace DefaultNamespace.BehaviorTree
+{
+    public class Tree : MonoBehaviour
+    {
+        private Node _rootNode;
+
+        private void OnEnable()
+        {
+            if (_rootNode == null) return;
+            _rootNode.Start();
+        }
+
+        private void Update()
+        {
+            if (_rootNode == null) return;
+            _rootNode.Update();
+        }
+
+        private void OnDisable()
+        {
+            if (_rootNode == null) return;
+            _rootNode.End();
+        }
+    }
+}