Add serializer(vararg KSerializer<*>) override from SerializerFactory when necessary

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

plugins/kotlinx-serialization/testData/boxIr/serializerFactoryInUserDefined.kt

@@ -0,0 +1,45 @@
+// WITH_STDLIB
+// ISSUE: KT-59768
+
+import kotlinx.serialization.*
+import kotlinx.serialization.json.*
+
+@Serializable sealed class SealedInterface {
+    companion object {
+        fun unrelated() {}
+    }
+}
+@Serializable enum class EnumKlass { INSTANCE;
+    companion object  {
+        fun unrelated() {}
+    }
+}
+
+@Serializable class Plain {
+    companion object  {
+        fun unrelated() {}
+    }
+}
+
+@MetaSerializable
+@Target(AnnotationTarget.CLASS, AnnotationTarget.PROPERTY)
+annotation class MySerializable
+
+@MySerializable
+sealed interface SealedMeta {
+    companion object  {
+        fun unrelated() {}
+    }
+}
+
+fun box(): String {
+    serializer<EnumKlass>()
+    EnumKlass.serializer()
+    serializer<SealedInterface>()
+    SealedInterface.serializer()
+    serializer<Plain>()
+    Plain.serializer()
+    serializer<SealedMeta>()
+    SealedMeta.serializer()
+    return "OK"
+}