feat: messages extension (#4028)

* feat: private messages

Author: SychO9

LICENSE

@@ -1,6 +1,6 @@
 The MIT License (MIT)
 
-Copyright (c) 2019-2021 Stichting Flarum (Flarum Foundation)
+Copyright (c) 2019-2024 Stichting Flarum (Flarum Foundation)
 Copyright (c) 2014-2019 Toby Zerner ([email protected])
 
 Permission is hereby granted, free of charge, to any person obtaining a copy