[improvement] Use scoped-slots in Vue 2.6+ (#2688)

packages/tabbar/index.js

@@ -53,7 +53,7 @@ export default sfc({
         style={{ zIndex: this.zIndex }}
         class={['van-hairline--top-bottom', bem({ fixed: this.fixed })]}
       >
-        {this.$slots.default}
+        {this.slots()}
       </div>
     );
   }