检测浏览器版本

检查浏览器，版本太低友好提示

const agent = navigator.userAgent.toLowerCase();
const regStr_ie = /msie [\d.]+;/gi;
const regStr_ff = /firefox\/[\d.]+/gi;
const regStr_chrome = /chrome\/[\d.]+/gi;
const regStr_saf = /safari\/[\d.]+/gi;