Add Gradle to Java library and CI to autopublish Maven package onto GitHub

[smyalygames]

Hiya,

I added Gradle to the Java library as I just wanted an easier way to build the package. I also added publishing to GitHub. It should be all set up and will run when you create a new release. :)

Changes I made:

• Add Gradle
• Refactored certain folders to match their package names
• Update Java tests to JUnit 5 (because it provides integration with Gradle)
• Add GitHub publishing CI
• Added .gitignores for Gradle and a few more IntelliJ files