message: refactor(DatetimePicker): refactor with composition api
diff: +import { ref } from 'vue';
  import { createNamespace } from '../utils';
  +import { usePublicApi } from '../composition/use-public-api';
  import TimePicker from './TimePicker';
  import DatePicker from './DatePicker';
  @@ -10,15 +12,16 @@ export default createComponent({
  ...DatePicker.props,
  },
  - methods: {
  - // @exposed-api
  - getPicker() {
  - return this.$refs.root.getPicker();
  - },
  - },
  + setup(props) {
  + const rootRef = ref();
  +
  + usePublicApi({
  + getPicker: () => rootRef.value && rootRef.value.getPicker(),
  + });
  - render() {
  - const Component = this.type === 'time' ? TimePicker : DatePicker;
  - return <Component ref="root" class={bem()} {...this.$props} />;
  + return () => {
  + const Component = props.type === 'time' ? TimePicker : DatePicker;
  + return <Component ref={rootRef} class={bem()} {...props} />;
  + };
  },
  });
  