See KT-21328
Compiler removes all files from its output directory.
Due to this reason (and some other reasons) it't not a good idea
to put DCE output to `classes` dir.
Therefore we changed output dir to `$buildDir/kotlin-js-min`.
Additionally, we now allow to customize output dir.
In particular, this fixes issues with webpack in multi-project build +
continuous Gradle build.
Alexey Andreev
1d48442a5d