input框根据光标位置插入数据

思路：获取光标位置，然后截取拼接字符串，重新填入input，重新设置光标位置

insertInportTxt (insertTxt) {
      let elInput = this.$refs.mytextarea.$el  //获取Dom
      let startPos = elInput.selectionStart
      let endPos = elInput.selectionEnd
      if (startPos === undefined || endPos === undefined) return
      let txt = elInput.value
      let result = txt.substring(0, startPos) + insertTxt + txt.substring(endPos)
      elInput.value = result
      elInput.focus()
      elInput.selectionStart = startPos + insertTxt.length
      elInput.selectionEnd = startPos + insertTxt.length
    }