Suggestion: Move to gradle and paper's userdev plugin

[Joo200]

While adding 1.21.3 support to the plugin I had some issues with the project setup. Debugging the project and adding new modules could be more comfortable.

I tried to switch the build tool from Maven to Gradle and use the gradle userdev plugin to setup the nms mappings.

The changes are tested with spigot and paper with the versions 1.21.3 and 1.21.4.

I'm not sure if you want to switch to gradle. If not I would keep that fork.

[DerFrZocker]

Thanks for the PR, however I'm currently only conferable using Maven and find it personally easier to use than gradle, with which I had my fair share of issues, when I tried it or it was used in other projects.
This however is probably only because I started with Maven and have therefore more experience in it and never really used gradle long enough to get to know it.

Consider your PR therefore to be stuck in limbo until I find the motivation time to fully learn gradle.