feat(Collapse): 新增外部样式类 title-class @rex-zsd (#1271)

packages/collapse-item/index.ts

@@ -1,7 +1,7 @@
 import { VantComponent } from '../common/component';
 
 VantComponent({
-  classes: ['content-class'],
+  classes: ['title-class', 'content-class'],
 
   relation: {
     name: 'collapse',

packages/collapse-item/index.wxml

@@ -3,6 +3,7 @@
 <view class="van-collapse-item van-hairline--top custom-class">
   <van-cell
     title="{{ title }}"
+    title-class="title-class"
     icon="{{ icon }}"
     is-link="{{ isLink }}"
     value="{{ value }}"