监听地图放大或者缩小，获取到放大的值或缩小的值
https://developers.weixin.qq.com/s/fywa6Zmw7nc8
bindregionchange
获取地图的缩放级别
https://developers.weixin.qq.com/miniprogram/dev/api/media/map/MapContext.getScale.html#%E5%8F%82%E6%95%B0
试一遍以下接口
