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/tests/org/jetbrains/kotlinx/serialization/TestGenerator.kt

@@ -64,6 +64,10 @@ fun main(args: Array<String>) {
             testClass<AbstractSerializationIrJsBoxTest> {
                 model("boxIr")
             }
+
+            testClass<AbstractSerializationFirJsBoxTest> {
+                model("boxIr")
+            }
         }
     }
 }

plugins/kotlinx-serialization/tests/org/jetbrains/kotlinx/serialization/runners/BoxTests.kt

@@ -5,6 +5,8 @@
 
 package org.jetbrains.kotlinx.serialization.runners
 
+import org.jetbrains.kotlin.js.test.fir.AbstractFirJsBoxTest
+import org.jetbrains.kotlin.js.test.fir.AbstractFirJsTest
 import org.jetbrains.kotlin.test.builders.TestConfigurationBuilder
 import org.jetbrains.kotlin.test.runners.codegen.AbstractFirLightTreeBlackBoxCodegenTest
 import org.jetbrains.kotlin.test.runners.codegen.AbstractIrBlackBoxCodegenTest
@@ -48,4 +50,14 @@ open class AbstractSerializationIrJsBoxTest : AbstractJsIrTest(
         super.configure(builder)
         builder.configureForKotlinxSerialization(target = TargetBackend.JS_IR)
     }
-}
+}
+
+open class AbstractSerializationFirJsBoxTest : AbstractFirJsTest(
+    pathToTestDir = "plugins/kotlinx-serialization/testData/boxIr/",
+    testGroupOutputDirPrefix = "codegen/serializationBoxFir/"
+) {
+    override fun configure(builder: TestConfigurationBuilder) {
+        super.configure(builder)
+        builder.configureForKotlinxSerialization(target = TargetBackend.JS_IR)
+    }
+}