push({ task, params = {} }) {
    const id = Utils.genUuid();
    const paramsWithId = { ...params, taskId: id };
    this.queue.push({ id, task, params: paramsWithId, state: 'waiting', file: null });
    return id;
  }
Add a subtask to the end of the task queue
@param {function} task - a task handler function
@return {string} task id
@public