Rewrite Gradle build scripts using Kotlin DSL and Architectury Loom

[object-Object]

The current Hex Casting Gradle setup is very old and ad-hoc, using different Gradle plugins for each platform (Sponge VanillaGradle, Fabric Loom, and ForgeGradle). We could probably replace all of these with Arch Loom.

Also, I would really like to rewrite the build scripts in Kotlin instead of Groovy - I've found it to be much more pleasant to work with, and seems to have better IDE support.