feat(Tabbar): add placeholder prop (#5979)

src/tabbar/test/__snapshots__/index.spec.js.snap

@@ -2,6 +2,12 @@
 
 exports[`disable border 1`] = `<div class="van-tabbar van-tabbar--fixed"></div>`;
 
+exports[`placeholder prop 1`] = `
+<div class="van-tabbar__placeholder" style="height: 50px;">
+  <div class="van-hairline--top-bottom van-tabbar van-tabbar--fixed"></div>
+</div>
+`;
+
 exports[`route mode 1`] = `
 <div class="van-hairline--top-bottom van-tabbar van-tabbar--fixed">
   <div class="van-tabbar-item van-tabbar-item--active">

src/tabbar/test/index.spec.js

@@ -1,5 +1,5 @@
 import VueRouter from 'vue-router';
-import { mount, later } from '../../../test';
+import { mount, later, mockGetBoundingClientRect } from '../../../test';
 import Vue from 'vue';
 import Tabbar from '..';
 
@@ -174,3 +174,18 @@ test('disable border', () => {
 
   expect(wrapper).toMatchSnapshot();
 });
+
+test('placeholder prop', () => {
+  const restore = mockGetBoundingClientRect({ height: 50 });
+
+  const wrapper = mount(Tabbar, {
+    propsData: {
+      fixed: true,
+      placeholder: true,
+    },
+  });
+
+  expect(wrapper).toMatchSnapshot();
+
+  restore();
+});