Add Gradle and GitHub Actions CI

[hamer0]

• Configured gradle for the project.
  • Kept the original structure of the project rather than moving over files to the recommended gradle file structure for now.
  • Similarly I left the /lib folder .jar files as local dependencies but these can probably be pulled from Maven Central in the future.
  • Moved the /tests folder into a parent /test folder since the test classes were in the package "tests". Gradle needs a root folder for test classes which can't be the root package (I might be wrong about this but it wasn't working without doing this).
• Added an Actions workflow to run Gradle build and test.