fix(messages): no message kind for :undo messages #31590

Problem: cannot handle `:undo` and `:redo` messages in a special way,
e.g. replace one by another.

Solution: add `undo` kind.

src/nvim/undo.c

@@ -1862,6 +1862,7 @@ static void u_doit(int startcount, bool quiet, bool do_buf_event)
     u_oldcount = -1;
   }
 
+  msg_ext_set_kind("undo");
   int count = startcount;
   while (count--) {
     // Do the change warning now, so that it triggers FileChangedRO when