Kotlin/JS configuration

This module, despite its name, for historical reasons contains a bunch of different stuff related to the Kotlin/JS compiler.

Right now here we can find:

• The JS AST protobuf serializer. This is no longer used anywhere in the compiler, but it is used by the IntelliJ IDEA metadata decompiler. To be removed (KT-73067).
• Some incremental compilation helpers (to be moved into some more appropriate place).