Gradle is better than Maven, don't @ me. okay but actually it's [faster](https://www.youtube.com/watch?v=atuFSv2bLa8&feature=youtu.be&t=77), compiles and tests in parallel more efficiently, and more epic stuff).
This commit adds Checkstyle rules for EssentialsX, based on those from the adventure library (https://github.com/KyoriPowered/adventure/tree/master/.checkstyle). It also makes Checkstyle passes mandatory for a GH build to pass.