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

src/image-preview/ImagePreview.js

@@ -249,12 +249,20 @@ export default createComponent({
       if (this.showIndex) {
         return (
           <div class={bem('index')}>
-            {this.slots('index') || `${this.active + 1}/${this.images.length}`}
+            {this.slots('index') || `${this.active + 1} / ${this.images.length}`}
           </div>
         );
       }
     },
 
+    genCover() {
+      const cover = this.slots('cover');
+
+      if (cover) {
+        return <div class={bem('cover')}>{cover}</div>;
+      }
+    },
+
     genImages() {
       const imageSlots = {
         loading: () => <Loading type="spinner" />
@@ -306,6 +314,7 @@ export default createComponent({
         <div class={[bem(), this.className]}>
           {this.genImages()}
           {this.genIndex()}
+          {this.genCover()}
         </div>
       </transition>
     );