fix(messages): list_cmd kind for :registers, :au[g] #32531

Problem:  No kind for `:registers/autocmd/augroup` messages. `:registers`
          chunks are emitted as separate `msg_show` events.

Solution: Add the `list_cmd` kind to the message. Introduce a new
          `msg_ext_skip_flush` variable to set to true around a
          group of to be paired message chunks.

src/nvim/message.c

@@ -3162,6 +3162,8 @@ void msg_ext_ui_flush(void)
   if (!ui_has(kUIMessages)) {
     msg_ext_kind = NULL;
     return;
+  } else if (msg_ext_skip_flush) {
+    return;
   }
 
   msg_ext_emit_chunk();