feat(ImagePreview): add cover slot (#4766)

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

@@ -1,5 +1,15 @@
 // Jest Snapshot v1, https://goo.gl/fbAQLP
 
+exports[`cover slot 1`] = `
+<div class="van-image-preview" name="van-fade">
+  <div class="van-swipe van-image-preview__swipe">
+    <div class="van-swipe__track" style="width: 0px; transition-duration: 0ms; transform: translateX(0px);"></div>
+  </div>
+  <div class="van-image-preview__index">1 / 0</div>
+  <div class="van-image-preview__cover">Custom Cover Content</div>
+</div>
+`;
+
 exports[`index slot 1`] = `
 <div class="van-image-preview" name="van-fade">
   <div class="van-swipe van-image-preview__swipe">
@@ -36,7 +46,7 @@ exports[`lazy-load prop 1`] = `
       </div>
     </div>
   </div>
-  <div class="van-image-preview__index">1/3</div>
+  <div class="van-image-preview__index">1 / 3</div>
 </div>
 `;
 
@@ -67,7 +77,7 @@ exports[`render image 1`] = `
       </div>
     </div>
   </div>
-  <div class="van-image-preview__index">1/3</div>
+  <div class="van-image-preview__index">1 / 3</div>
 </div>
 `;
 
@@ -106,6 +116,6 @@ exports[`zoom 1`] = `
       </div>
     </div>
   </div>
-  <div class="van-image-preview__index">1/3</div>
+  <div class="van-image-preview__index">1 / 3</div>
 </div>
 `;

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

@@ -151,7 +151,19 @@ test('index slot', () => {
   const wrapper = mount({
     template: `
       <van-image-preview :value="true">
-        <template v-slot:index>Custom Index</template>
+        <template #index>Custom Index</template>
+      </van-image-preview>
+    `
+  });
+
+  expect(wrapper).toMatchSnapshot();
+});
+
+test('cover slot', () => {
+  const wrapper = mount({
+    template: `
+      <van-image-preview :value="true">
+        <template #cover>Custom Cover Content</template>
       </van-image-preview>
     `
   });