vue2 el-menu 箭头改成el-tree风格，旋转180°改成90°

转成下面风格

//菜单关闭
/deep/ .el-submenu > .el-submenu__title .el-submenu__icon-arrow {
  -webkit-transform: rotateZ(-90deg);
  -ms-transform: rotate(-90deg);
  transform: rotateZ(-90deg);
}

//菜单展开
/deep/ .el-submenu.is-opened > .el-submenu__title .el-submenu__icon-arrow {
  -webkit-transform: rotateZ(0deg);
  -ms-transform: rotate(0deg);
  transform: rotateZ(0deg);
}

.el-submenu {
  .el-menu-item {
    padding-left: 40px !important;
  }

  /deep/ .el-submenu__icon-arrow {
    left: 0 !important;
    right: auto !important;
    top: 55% !important;
  }

  /deep/ .el-icon-arrow-down:before {
    content: '\e790' !important; //这个就是三角形
  }
}