message: docs(Tabbar): use composition api
diff: @@ -25,11 +25,12 @@ app.use(TabbarItem);
  ```
  ```js
  +import { ref } from 'vue';
  +
  export default {
  - data() {
  - return {
  - active: 0,
  - };
  + setup() {
  + const active = ref(0);
  + return { active };
  },
  };
  ```
  @@ -46,11 +47,12 @@ export default {
  ```
  ```js
  +import { ref } from 'vue';
  +
  export default {
  - data() {
  - return {
  - active: 'home',
  - };
  + setup() {
  + const active = ref('home');
  + return { active };
  },
  };
  ```
  @@ -84,14 +86,18 @@ Use `icon` slot to custom icon.
  ```
  ```js
  +import { ref } from 'vue';
  +
  export default {
  - data() {
  - return {
  - active: 0,
  - icon: {
  + setup() {
  + const active = ref(0);
  + const icon = {
  active: 'https://img.yzcdn.cn/vant/user-active.png',
  inactive: 'https://img.yzcdn.cn/vant/user-inactive.png',
  - },
  + };
  + return {
  + icon,
  + active,
  };
  },
  };
  @@ -120,13 +126,20 @@ export default {
  ```
  ```js
  -import { Notify } from 'vant';
  +import { ref } from 'vue';
  +import { Toast } from 'vant';
  export default {
  - methods: {
  - onChange(index) {
  - Notify({ type: 'primary', message: index });
  - },
  + setup() {
  + const active = ref(0);
  + const onChange = (index) => {
  + Toast(`Tab ${index}`);
  + };
  +
  + return {
  + icon,
  + onChange,
  + };
  },
  };
  ```
  