Switch Kotlin to new IR backend

New backend in Kotlin 1.4:

• must be explicitely enabled : JVM: [build.gradle.kts] kotlinOptions.useIR = true JS: [gradle.properties] kotlin.js.compiler=ir or [build.gradle.kts] kotlin { js(IR) … }
• generate TS definition files (.d.ts)
• only export to JS declarations annotated with the new @JsExport
• generated JVM code is not compatible with old backend code ; consider providing both.