I forked the project on SourceForge, but seems like a lot of work, don't know if I will get around to doing it (as I have other projects I'm working on).
I found this: https://www.scintilla.org/Lexer.txt
Basically, you would need to copy this file as LexKotlin.cxx: https://github.com/geany/geany/blob/master/scintilla/lexers/LexCPP.cxx https://sourceforge.net/p/scintilla/code/ci/default/tree/lexers/LexCPP.cxx
Then add the new keywords and syntax that Kotlin has, such as `object` (lower-case), `data class`, `sealed class`, `fun`, `then`, etc. I think you'd probably also want to remove parsing of the ternary operator `a ? b : c` and add parsing for the elvis operator `a ?: b`.
You can also look at this project that is adding the Rust language: https://sourceforge.net/u/dobkeratops/scintilla/ci/default/tree/
Here are other open source projects that have added Kotlin support for help as well: https://github.com/vkostyukov/kotlin-sublime-package https://github.com/nishtahir/language-kotlin