Prepare code for Kotlin Multiplatform

[aSemy]

Part of #57

Summary

• Update Shadow to new updated fork (the previous Shadow was deprecated).
• Replace JUnit with Kotlin Test (so that the tests can be moved into a common Kotlin source set).
• Remove JVM specific code and utilities, and replace them with pure Kotlin code.
• Update Gradle to 8.9.
• Configure buildSrc convention plugins, for easier build config management.
• Disable ktlint. v11 reports several nonsensical, unresolvable style errors. v12 reports hundreds of errors.