v-for json渲染三层

   navlist: [
        {
          label: "一级 1",
          children: [
            {
              label: "二级 1-1",
              children: [
                {
                  label: "三级 1-1-1"
                }
              ]
            }
          ]
        },
        {
          label: "一级 2",
          children: [
            {
              label: "二级 2-1",
              children: [
                {
                  label: "三级 2-1-1"
                }
              ]
            },
            {
              label: "二级 2-2",
              children: [
                {
                  label: "三级 2-2-1"
                }
              ]
            }
          ]
        },
        {
          label: "一级 3",
          children: [
            {
              label: "二级 3-1",
              children: [
                {
                  label: "三级 3-1-1"
                }
              ]
            },
            {
              label: "二级 3-2",
              children: [
                {
                  label: "三级 3-2-1"
                }
              ]
            }
          ]
        }
      ]