msglist: Use directional positioning for unread marker in RTL.

Author: yash-agarwa-l

lib/widgets/message_list.dart

@@ -1639,9 +1639,9 @@ class _UnreadMarker extends StatelessWidget {
     return Stack(
       children: [
         child,
-        Positioned(
+        PositionedDirectional(
           top: 0,
-          left: 0,
+          start: 0,
           bottom: 0,
           width: 4,
           child: AnimatedOpacity(
@@ -1653,13 +1653,20 @@ class _UnreadMarker extends StatelessWidget {
             child: DecoratedBox(
               decoration: BoxDecoration(
                 color: messageListTheme.unreadMarker,
-                border: Border(left: BorderSide(
+                border: BorderDirectional(start: BorderSide(
                   width: 1,
-                  color: messageListTheme.unreadMarkerGap)))))),
-      ]);
+                  color: messageListTheme.unreadMarkerGap,
+                )),
+              ),
+            ),
+          ),
+        ),
+      ],
+    );
   }
 }
 
+
 class StreamMessageRecipientHeader extends StatelessWidget {
   const StreamMessageRecipientHeader({
     super.key,