I got something working. I’ll try this on android template later (edit: apologies, I have no idea how to create a Koltin android project without android studio, but you can probably generate one with android studio then upload it onto IDX, maybe)
packages = [
pkgs.kotlin
pkgs.gradle
pkgs.jdk # OR (maybe) pkgs.temurin-bin
];
idx = {
extensions = [
"fwcd.kotlin"
];
}