Is it yet possible to easily include Kotlin code inside of a JS/TS project and have everything just work? Specifically one that doesn't use Gradle but just uses Webpack (or a different bundler) directory? I'd love to have a way to just have a directory full of Kotlin code, point Webpack at it, and get back a bundled JS file with a Typescript declaration file next to it.

Nope, that's never a goal for kjs. You need either gradle or kotlinc to convert kotlin code into usable js code.