[new feature] ImagePreview: add index slot (#3157)

packages/image-preview/demo/index.vue

@@ -17,7 +17,10 @@
       <van-image-preview
         v-model="show"
         :images="images"
-      />
+        @change="onChange"
+      >
+        <template v-slot:index>{{ $t('index', index) }}</template>
+      </van-image-preview>
     </demo-block>
   </demo-section>
 </template>
@@ -38,20 +41,23 @@ export default {
       button1: '预览图片',
       button2: '指定初始位置',
       button3: '异步关闭',
-      componentCall: '组件调用'
+      componentCall: '组件调用',
+      index: index => `第${index + 1}页`
     },
     'en-US': {
       button1: 'Show Images',
       button2: 'Custom Start Position',
       button3: 'Async Close',
-      componentCall: 'Component Call'
+      componentCall: 'Component Call',
+      index: index => `Page: ${index}`
     }
   },
 
   data() {
     return {
       show: false,
-      images
+      images,
+      index: 0
     };
   },
 
@@ -60,6 +66,10 @@ export default {
       this.show = true;
     },
 
+    onChange(index) {
+      this.index = index;
+    },
+
     showImagePreview(position, timer) {
       const instance = ImagePreview({
         images,