feat(Dialog): add opened、closed event (#4634)

src/dialog/Dialog.js

@@ -73,6 +73,14 @@ export default createComponent({
       if (this.callback) {
         this.callback(action);
       }
+    },
+
+    onOpened() {
+      this.$emit('opened');
+    },
+
+    onClosed() {
+      this.$emit('closed');
     }
   },
 
@@ -131,7 +139,11 @@ export default createComponent({
     );
 
     return (
-      <transition name="van-dialog-bounce">
+      <transition
+        name="van-dialog-bounce"
+        onAfterEnter={this.onOpened}
+        onAfterLeave={this.onClosed}
+      >
         <div
           vShow={this.value}
           role="dialog"