H5 app安全区适配

<meta name="viewport" content="..., viewport-fit=cover" />

/* 适配iphoneX顶部填充*/
@supports (top: env(safe-area-inset-top)){
  body,
  .header{
      padding-top: constant(safe-area-inset-top);
      padding-top: env(safe-area-inset-top);
  }
}
/* 判断iphoneX 将 footer 的 padding-bottom 填充到最底部 */
@supports (bottom: env(safe-area-inset-bottom)){
    body,
    .footer{
        padding-bottom: constant(safe-area-inset-bottom);
        padding-bottom: env(safe-area-inset-bottom);
    }
}