code: function focusNav (elem, step) {
    let focusable = getFocusable(document.body)
    let index = findIndex(focusable, el => el === elem)
    if (index !== -1) {
      let next = focusable[index + step]
      if (next) {
        next.focus()
      }
    }
  }
docs: 聚焦到前/后第指定个可聚焦元素
  
  @param {HTMLElement} elem 起始元素
  @param {number} step 偏移量