微信小程序PC端padStart方法不支持

微信小程序PC端padStart方法不支持，改为如下

const now = new Date()
const day = ("0" + now.getDate()).slice(-2);
const month = ("0" + (now.getMonth() + 1)).slice(-2);
 //拼装日期格式  
var date = now.getFullYear() + "-" + now.getMonth() + 1 + "-" + now.getDate()