feat(NavBar): add placeholder prop (#5938)

src/nav-bar/demo/index.vue

@@ -11,10 +11,10 @@
       />
     </demo-block>
 
-    <demo-block :title="t('advancedUsage')">
+    <demo-block :title="t('useSlot')">
       <van-nav-bar :title="t('title')" :left-text="t('back')" left-arrow>
         <template #right>
-          <van-icon name="search" />
+          <van-icon name="search" size="18" />
         </template>
       </van-nav-bar>
     </demo-block>
@@ -23,6 +23,15 @@
 
 <script>
 export default {
+  i18n: {
+    'zh-CN': {
+      useSlot: '使用插槽',
+    },
+    'en-US': {
+      useSlot: 'Use Slot',
+    },
+  },
+
   methods: {
     onClickLeft() {
       this.$toast(this.t('back'));