Update Overcooked simulator to support state export in JSON format

The update introduces a new function `get_json_state()` in overcooked_environment.py that exports the current game state in JSON format. Changes were made to player.py, counters.py, and game_items.py to include an additional `to_dict()` function which represents their current status as a dictionary. This commit also includes minor naming changes, such as renaming `beginning_time` to `start_time`.