Kotlin/JVM & GraalVM playground

This playgound includes a Nix flake providing the GraalVM Native Image toolchain to produce Kotlin/JVM executables.

These executables are currently built using the official Gradle plugin from GraalVM.

In a future iteration, we'll attempt to build them with these Bazel rules:

• bazelbuild/rules_kotlin
• sgammon/rules_graalvm