[improvement] add constant of border (#4078)

src/number-keyboard/index.js

@@ -1,5 +1,6 @@
 import { createNamespace } from '../utils';
 import { stopPropagation } from '../utils/dom/event';
+import { BORDER_TOP } from '../utils/constant';
 import { BindEventMixin } from '../mixins/bind-event';
 import Key from './Key';
 
@@ -137,7 +138,7 @@ export default createComponent({
     const showTitle = title || showTitleClose || titleLeftSlot;
 
     const Title = showTitle && (
-      <div class={[bem('title'), 'van-hairline--top']}>
+      <div class={[bem('title'), BORDER_TOP]}>
         {titleLeftSlot && <span class={bem('title-left')}>{titleLeftSlot}</span>}
         {title && <span>{title}</span>}
         {showTitleClose && (