Adding support for 1.21 for the spigot part of the plugin

Changing internal structure to get the bungeecord part work with velocity in combination with Snap.

This update adjusts the internal structure of the plugin.

The update also increases the support for the spigot part of the plugin to spigot 1.18. So you can use the spigot function on spigot 1.8 and 1.18.

This update changes the spigot messaging channel identifier. So when updating keep in mind to replace the old version everywhere. There is no backwards compatiblity.

The update also increases the support for the spigot part of the plugin to spigot 1.18. So you can use the spigot function on spigot 1.8 and 1.18.

This version has a function to make a confirmation with a book. Therefore you can install this plugin on the bungeecord server and on the spigot server and the inventory will be displayed accordingly.

Currently the Spigot part is only for the 1.8, but Bungeecord still works in all versions.