Add z-index to tabbar for proper stacking context (#364)

src/tabbar/index.vue

@@ -145,7 +145,8 @@ function getImageByIndex(index: number, item: CustomTabBarItem) {
   bottom: 0;
   left: 0;
   right: 0;
-
+  z-index: 1000;
+  
   border-top: 1px solid #eee;
   box-sizing: border-box;
 }