Build infra overhaul - Githubissues

This PR is meant to restore releasabiLITI of the engine by:

restructuring the Gradle project
splitting build scripts into reusable components
once again removing Kotlin Gradle scripts in favor of simplified Groovy scripts
removing some explicit Gradle configurations and plugins in favor of using defaults
refining dependency management
updating dependencies
reconfiguring the publishing pipeline
future-proofing our versioning scheme by applying semantic versioning
properly using GitHub actions to sign and release artifacts

gurkenlabs / litiengine