Use Gradle instead for building the plugin

[mrjasonn]

Gradle is faster and better, so it would make sense to use it if feasible.

[frengor]

Gradle would be an improvement, however Maven is working just fine, so for us moving to Gradle is not worth it at the moment. Also, considering we have not much free time, we'd prefer to work on new features rather than moving to Gradle.

(Of course, contributions are welcomed!)

[HydrolienF]

If the project switch to Gradle, it would be nice to use paperweight instead of local spigot sources so that new contributors of this project don't need to build the 17 spigot build one by one from the spigot build tool.
With paperweight, all the needed compilation is done by gradle on the first build.