[new feature] Tabs: add nav-left、nav-right slots (#2828)

packages/tab/test/__snapshots__/index.spec.js.snap

@@ -139,6 +139,28 @@ exports[`lazy render 2`] = `
 </div>
 `;
 
+exports[`render nav-left & nav-right slot 1`] = `
+<div class="van-tabs van-tabs--line">
+  <div class="van-tabs__wrap van-hairline--top-bottom">
+    <div class="van-tabs__nav van-tabs__nav--line" style="border-color: #f44;">Nav Left<div class="van-tabs__line" style="background-color: rgb(255, 68, 68);"></div>
+      <div class="van-tab van-tab--active"><span class="van-ellipsis">title1</span></div>
+      <div class="van-tab"><span class="van-ellipsis"></span></div>
+      <div class="van-tab van-tab--disabled"><span class="van-ellipsis">title3</span></div>Nav Right
+    </div>
+  </div>
+  <div class="van-tabs__content">
+    <div class="van-tab__pane" style="">Text</div>
+    <div class="van-tab__pane" style="display: none;">
+      <!---->
+      <div><span>title2</span></div>
+    </div>
+    <div class="van-tab__pane" style="display: none;">
+      <!---->
+    </div>
+  </div>
+</div>
+`;
+
 exports[`swipe to switch tab 1`] = `
 <div class="van-tabs van-tabs--line">
   <div class="van-tabs__wrap van-hairline--top-bottom">

packages/tab/test/index.spec.js

@@ -2,17 +2,19 @@ import Tab from '..';
 import Tabs from '../../tabs';
 import { mount, later, triggerDrag } from '../../../test/utils';
 
-function createWrapper(options) {
+function createWrapper(options = {}) {
   return mount({
     template: `
-      <tabs @change="onChange"
+      <tabs
         :color="color"
         :type="type"
-        :swipeable="swipeable"
         :sticky="sticky"
+        :swipeable="swipeable"
         :line-width="lineWidth"
         :lazy-render="lazyRender"
+        @change="onChange"
       >
+        ${options.extraTemplate || ''}
         <tab :title="title1">Text</tab>
         <tab>
           <span slot="title">title2</span>
@@ -112,3 +114,14 @@ test('lazy render', async () => {
   await later();
   expect(wrapper).toMatchSnapshot();
 });
+
+test('render nav-left & nav-right slot', async () => {
+  const wrapper = createWrapper({
+    extraTemplate: `
+      <template v-slot:nav-left>Nav Left</template>
+      <template v-slot:nav-right>Nav Right</template>
+    `
+  });
+
+  expect(wrapper).toMatchSnapshot();
+});