getCollidables() {
    const collidables = this.draggable.options.collidables;

    if (typeof collidables === 'string') {
      return Array.prototype.slice.call(document.querySelectorAll(collidables));
    } else if (
      collidables instanceof NodeList ||
      collidables instanceof Array
    ) {
      return Array.prototype.slice.call(collidables);
    } else if (collidables instanceof HTMLElement) {
      return [collidables];
    } else if (typeof collidables === 'function') {
      return collidables();
    } else {
      return [];
    }
  }
Returns current collidables based on `collidables` option
@return {HTMLElement[]}