feat(Stepper): add theme prop (#6282)

* feat(Stepper): add theme prop

* docs: add round theme doc

src/stepper/demo/index.vue

@@ -35,6 +35,15 @@
     <van-cell center :title="t('asyncChange')">
       <van-stepper :value="stepper6" async-change @change="onChange" />
     </van-cell>
+
+    <van-cell v-if="!isWeapp" center :title="t('roundTheme')">
+      <van-stepper
+        v-model="stepperRound"
+        theme="round"
+        button-size="22"
+        disable-input
+      />
+    </van-cell>
   </demo-section>
 </template>
 
@@ -45,6 +54,7 @@ export default {
       step: '步长设置',
       range: '限制输入范围',
       integer: '限制输入整数',
+      roundTheme: '圆角风格',
       asyncChange: '异步变更',
       customSize: '自定义大小',
       disableInput: '禁用输入框',
@@ -54,6 +64,7 @@ export default {
       step: 'Step',
       range: 'Range',
       integer: 'Integer',
+      roundTheme: 'Round Theme',
       asyncChange: 'Async Change',
       customSize: 'Custom Size',
       disableInput: 'Disable Input',
@@ -71,6 +82,7 @@ export default {
       stepper6: 1,
       stepper7: 1,
       stepper8: 1,
+      stepperRound: 1,
       disabledInput: 1,
     };
   },