Add the ability to modify menu data before saving

Hi! I tried this module because book_tree_menu has issues with Drupal 10 and seems no longer maintained. This module worked perfectly for my needs. However, I wanted a way to modify the data, and I found no event in the module that would let me hook in and tweak things. So I introduced an action that triggers an event. I think it’s a nice enhancement, since now you can simply subscribe to the event and modify the data as needed.