Prepare support Folia

[Euphillya]

I started working on Folia support for Citizens a few weeks ago.

It’s quite a substantial task, but I’ve made a good start and I’m making steady progress with the time I have available alongside my other projects.

I’m already submitting a preview of the changes that need to be made to the API.

I’ve maintained compatibility with Spigot, which was essential.

I’m leaving this PR as a draft, as I may still make changes if anything needs to be adjusted.