feat(core): use ListView.separated in FlatFeedCore

Author: GroovinChip

packages/stream_feed_flutter_core/lib/src/flat_feed_core.dart

@@ -113,10 +113,11 @@ class _GenericFlatFeedCoreState<A, Ob, T, Or>
         if (activities.isEmpty) {
           return widget.onEmptyWidget;
         }
-        return ListView.builder(
+        return ListView.separated(
           physics:
               widget.scrollPhysics ?? const AlwaysScrollableScrollPhysics(),
           itemCount: activities.length,
+          separatorBuilder: (context, index) => const Divider(),
           itemBuilder: (context, idx) => widget.feedBuilder(
             context,
             activities,