vant tab标题选中状态样式设计

覆盖原生样式：
1、将下划线变更为椭圆背景
2、选中项字体变粗

/deep/.van-tabs__line {
    position: absolute;
    bottom: 15px;
    left: 0;
    z-index: 1;
    min-width: 50px;
    height: 44px;
    padding: 0 10px;
    background-color: #dddddd47;
    border-radius: 30px;
    box-sizing: content-box;
}
/deep/.van-tab--active{
    font-weight: bold;
}