Upgrade Kotlin/JS Legacy compiler backend to the new version

[obask]

This project currently uses the Kotlin/JS Legacy compiler backend, which has been deprecated and will be removed in a future release.
Please migrate the project to the new IR-based compiler (https://kotl.in/jsir) by adding:
- kotlin.js.compiler=ir to your gradle.properties file.
- js(IR) { ... } to your build file.
You can continue to use the deprecated legacy compiler in the current version of the toolchain by specifying it explicitly.

[JSMonk]

#24