OfflineManager

Now the plugin supports not only 1.16, but also 1.12 versions.

Now the configuration files will update themselves without changing the previous configuration fields.

Updated the plugin API a bit. Added new directories and some classes for new and future features.

Updated the commands code. Optimized imports and added more documentation to API classes. I also added a check for server version support, so as not to cause problems in its operation if the plugin and server versions are incompatible.