v0.20.0-api4: Final release for SpongeAPI 4.x

After a lot of thought, and with the official release of SpongeAPI 5.0.0, I have decided to drop feature support for API 4.

This release contains all the feature updates that I have made up to this point. From this point on, features will be developed for API 5.
Similar to how things were, API 5 will be the API by which I write featues, with a forward port to API 6 (also known as bleeding).

Features will NOT be backported to API 4, but I may still include bugfixes if the need arises.

Thank you all for understanding, Now onto changes:

- FoxGuard can finally figure out who caused an explosion. Yay!
- There are now group handlers. This means that FoxGuard can finally hook into permissions again. Also yay!
- Added the first controller: The logic controller.
- The debug handler is actually usable and useful now. It just needs some documentation.
- Lots and lots of bugfixes.
- Probably other features that I completely forgot about.

Most of the updates were backend updates for future features, so I apologize that they couldn't make it into this release.

Anyway thanks for sticking with me all this time, and I hope you guys can enjoy the future updates!