Update min Dart constraint to 3.8, which will enable us to use the fancier collection literals. This requires bumping the min Flutter version as well: <img width="1892" height="1122" alt="image" src="https://github.com/user-attachments/assets/7c7b07fc-4d96-4987-824d-9a7133ecfb85" />
behavior_tree
flame_behavior_tree
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
