Kotlin conversion

[tajobe]

The plugin loads without error, though it's largely untested. Will leave as WIP in the title until some testing can be done.

A few other things to consider:

• the size of the shadowed jar is a bit larger with the kotlin standard lib included, though this can be minimized by adding a task that excludes the kotlin libraries (which would mean the standard lib would need to be on the classpath) Updated to use the 1.17+ libraries plugin feature: https://www.spigotmc.org/wiki/plugin-yml/#optional-attributes
• the plugin's configuration is mostly still todo... I didn't want to convert the existing ConfigObject class to Kotlin considering the reflection used and it really needs to be replaced instead Removed old configuration, loads into a kotlin data class
• I added some extension functions and cleaned up some of the more dense code to be more readable, so it's a bit more than a 1-to-1 conversion from Java to Kotlin
• I'm not sure why this was using the java-library plugin instead of java, but that's been replaced with the kotlin plugin. Let me know if that was erroneous