Add GitHub actions

[EinDev]

This will add the ability to:

• Compile all the code on GitHub's side
• Download the artifact on GitHub's website
• Ensure that it can be compiled on every commit, every change
• Ensure that the Core can be compiled against every version of Spigot

See my fork for examples, on the "actions" tab

[Chris6ix]

1.18.2 is no longer a thing and some dependencies need a bump