[improvement] ImagePreview: add show-index prop (#1889)

packages/image-preview/test/__snapshots__/index.spec.js.snap

@@ -2,6 +2,7 @@
 
 exports[`render image 1`] = `
 <div class="van-image-preview">
+  <div class="van-image-preview__index">1/3</div>
   <div class="van-swipe">
     <div class="van-swipe__track" style="width: 0px;">
       <div class="van-swipe-item" style="width: 0px; height: 100%;">

packages/image-preview/test/index.spec.js

@@ -1,6 +1,6 @@
 import Vue from 'vue';
 import ImagePreview from '..';
-import ImagePreviewVue from '../image-preview';
+import ImagePreviewVue from '../ImagePreview';
 import { mount, triggerDrag } from '../../../test/utils';
 
 const images = [