Rename Serializers.kt to SerializersJs.kt to support JS per-file compilation (#2315)

With the per-file compilation we use near the same approach as JVM target: we compiler the file in path of its package name + file name, so, if we had a Foo.kt file under the org.bar.baz package, the compiler result of this file will be placed in the build directory by org/bar/baz/Foo.mjs path.

Author: JSMonk