fix tab insert index (#2898)

packages/tabs/src/tabs.vue

@@ -60,7 +60,8 @@
         this.$emit('input', value);
       },
       addPanes(item) {
-        this.panes.push(item);
+        const index = this.$slots.default.indexOf(item.$vnode);
+        this.panes.splice(index, 0, item);
       },
       removePanes(item) {
         const panes = this.panes;