[improvement] add constant of border (#4078)

src/sidebar/index.js

@@ -1,5 +1,6 @@
 import { createNamespace } from '../utils';
 import { ParentMixin } from '../mixins/relation';
+import { BORDER_TOP_BOTTOM } from '../utils/constant';
 
 const [createComponent, bem] = createNamespace('sidebar');
 
@@ -18,6 +19,6 @@ export default createComponent({
   },
 
   render() {
-    return <div class={[bem(), 'van-hairline--top-bottom']}>{this.slots()}</div>;
+    return <div class={[bem(), BORDER_TOP_BOTTOM]}>{this.slots()}</div>;
   }
 });