feat(Area): add columns-placeholder prop (#4566)

src/area/demo/index.vue

@@ -18,6 +18,14 @@
         :title="$t('title')"
       />
     </demo-block>
+
+    <demo-block :title="$t('title4')">
+      <van-area
+        :area-list="$t('areaList')"
+        :columns-placeholder="$t('columnsPlaceholder')"
+        :title="$t('title')"
+      />
+    </demo-block>
   </demo-section>
 </template>
 
@@ -30,12 +38,16 @@ export default {
     'zh-CN': {
       title2: '选中省市区',
       title3: '配置显示列',
-      areaList: AreaList
+      title4: '配置列占位提示文字',
+      columnsPlaceholder: ['请选择'],
+      areaList: AreaList,
     },
     'en-US': {
       title2: 'Initial Value',
       title3: 'Columns Number',
-      areaList: AreaListEn
+      title4: 'Columns Placeholder',
+      columnsPlaceholder: ['Choose'],
+      areaList: AreaListEn,
     }
   },