getLastTabbableElement = el => {
    const tabbables = getTabbables([...el.children]);
    return tabbables.length ? tabbables[tabbables.length - 1] : null;
  }
Returns the last tabbable element within the provided HTMLElement.

@public
@param { HTMLElement } el the component to operate on (component that slots or contains within its shadow root the items the user navigates among)
@returns { HTMLElement | null } the last tabbable element or "null" if not found