feat: Add initial version of behavior_tree and flame_behavior_tree package (#3045)

First pass on adding behavior tree for flame. This PR adds 2 packages:

- behavior_tree: A pure dart implementation of behavior tree.
- flame_behavior_tree: A bridge package that integrates behavior_tree
with flame.

Demo: 


https://github.com/flame-engine/flame/assets/33748002/1d2b00ab-1b6e-406e-9052-a24370c8f1ab

packages/flame_behavior_tree/lib/src/has_behavior_tree.dart

@@ -0,0 +1,57 @@
+import 'dart:async';
+
+import 'package:behavior_tree/behavior_tree.dart';
+import 'package:flame/components.dart';
+import 'package:flutter/foundation.dart';
+
+/// A mixin on [Component] to indicate that the component has a behavior tree.
+///
+/// Reference to the behavior tree for this component can be set or accessed
+/// via [treeRoot]. The update frequency of the tree can be reduced by
+/// increasing [tickInterval]. By default, the tree will be updated on every
+/// update of the component.
+mixin HasBehaviorTree<T extends NodeInterface> on Component {
+  T? _treeRoot;
+  Timer? _timer;
+  double _tickInterval = 0;
+
+  /// The delay between any two ticks of the behavior tree.
+  double get tickInterval => _tickInterval;
+  set tickInterval(double interval) {
+    _tickInterval = interval;
+
+    if (_tickInterval > 0) {
+      _timer ??= Timer(interval, repeat: true);
+      _timer?.limit = interval;
+    } else {
+      _timer?.onTick = null;
+      _timer = null;
+      _tickInterval = 0;
+    }
+  }
+
+  /// The root node of the behavior tree.
+  T get treeRoot => _treeRoot!;
+  set treeRoot(T value) {
+    _treeRoot = value;
+    _timer?.onTick = _treeRoot!.tick;
+  }
+
+  @override
+  @mustCallSuper
+  Future<void> onLoad() async {
+    super.onLoad();
+    _timer?.onTick = _treeRoot?.tick;
+  }
+
+  @override
+  @mustCallSuper
+  void update(double dt) {
+    super.update(dt);
+    if (_tickInterval > 0) {
+      _timer?.update(dt);
+    } else {
+      _treeRoot?.tick();
+    }
+  }
+}