types(Button): add ButtonNativeType、ButtonIconPosition type (#9604)

packages/vant/src/button/README.md

@@ -154,7 +154,12 @@ app.use(Button);
 The component exports the following type definitions:
 
 ```ts
-import type { ButtonType, ButtonSize } from 'vant';
+import type {
+  ButtonType,
+  ButtonSize,
+  ButtonNativeType,
+  ButtonIconPosition,
+} from 'vant';
 ```
 
 ## Theming

packages/vant/src/button/README.zh-CN.md

@@ -177,7 +177,12 @@ app.use(Button);
 组件导出以下类型定义：
 
 ```ts
-import type { ButtonType, ButtonSize } from 'vant';
+import type {
+  ButtonType,
+  ButtonSize,
+  ButtonNativeType,
+  ButtonIconPosition,
+} from 'vant';
 ```
 
 ## 主题定制

packages/vant/src/button/index.ts

@@ -3,4 +3,9 @@ import _Button from './Button';
 
 export const Button = withInstall(_Button);
 export default Button;
-export type { ButtonType, ButtonSize } from './Button';
+export type {
+  ButtonType,
+  ButtonSize,
+  ButtonNativeType,
+  ButtonIconPosition,
+} from './Button';