Update chat.dart

Changed the definition for the bubbleBuilder.  It now has a defaultBubbleMessage which is the original message.

lib/src/widgets/chat.dart

@@ -116,6 +116,7 @@ class Chat extends StatefulWidget {
     Widget child, {
     required types.Message message,
     required bool nextMessageInGroup,
+    required Widget defaultBubbleMessage,
   })? bubbleBuilder;
 
   /// See [Message.bubbleRtlAlignment].