As of Crucible version 4.2.1, there is no built-in syntax highlight support for Kotlin files. I have already filled a feature request for that.
However, for now, the only possibility is to add a custom syntax config as described here.
Has anyone already done it and can share the config?
I ended up creating this file by myself. It's based on Java config but uses Kotlin keywords.
This project contains also a script to generate a prefix tree of keywords, if you don't like the list I'm using.
Below you can find the file's contents: