SerializerFactory is an implementation detail for Kotlin/JS and Native:
it should be added as a supertype to a companion object of certain serializable classes
and `serializer(vararg KSerializer<*>)` function from it should be implemented.
Existing implementation added the supertype, but did not add proper override to FirClass
of a companion, which led to various warnings and errors like 'Abstract function 'serializer' is not implemented in non-abstract companion object'.
Also implemented the addition of SerializerFactory supertype to user-defined companions within @Serializable and @MetaSerializable when necessary.
Also set up proper box tests for FIR+Kotlin/JS combination.
#KT-58501 Fixed
#KT-59768 Fixed
Leonid Startsev