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
}
