messageList: Add notification group

This add a widget for the new notification grouping
according to the new designs [1], but the expand behavior will be added
in a future commit.

[1] 9e2bed6f37/notifications-calendar/notifications-grouping.png

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3012>

data/theme/gnome-shell-sass/widgets/_message-list.scss

@@ -58,6 +58,19 @@
   }
 }
 
+// message notification group
+.message-notification-group {
+  spacing: $base_padding * 2;
+
+  .message-group-header {
+    padding: $base_padding;
+    .message-group-title {
+      @extend %title_2;
+      margin: 0 $base_margin;
+    }
+  }
+}
+
 // message bubbles
 .message {
   @extend %card;