style(Button): adjust font-smoothing

packages/vant/docs/markdown/changelog-v4.zh-CN.md

@@ -22,3 +22,4 @@
 
 - Button: 默认圆角大小从 `2px` 调整为 `4px`
 - Button: 默认按钮的边框颜色调整为 `--van-gray-6`
+- Button: 调整 font-smoothing，默认使用粗体文字

packages/vant/src/button/index.less

@@ -49,6 +49,7 @@
   cursor: pointer;
   transition: opacity var(--van-duration-fast);
   -webkit-appearance: none;
+  -webkit-font-smoothing: auto;
 
   &::before {
     position: absolute;