feat(field): add new external class label-class (#3311)

fix #3285

packages/field/README.md

@@ -260,5 +260,6 @@ Page({
 
 | 类名             | 说明           |
 | ---------------- | -------------- |
+| label-class      | 左侧文本样式类 |
 | input-class      | 输入框样式类   |
 | right-icon-class | 右侧图标样式类 |

packages/field/index.ts

@@ -6,7 +6,7 @@ import { canIUseModel } from '../common/version';
 VantComponent({
   field: true,
 
-  classes: ['input-class', 'right-icon-class'],
+  classes: ['input-class', 'right-icon-class', 'label-class'],
 
   props: {
     ...commonProps,

packages/field/index.wxml

@@ -14,6 +14,7 @@
   custom-style="{{ customStyle }}"
   arrow-direction="{{ arrowDirection }}"
   custom-class="van-field"
+  title-class="label-class"
 >
   <slot name="left-icon" slot="icon" />
   <slot name="label" slot="title" />