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-gen/org/jetbrains/kotlinx/serialization/runners/SerializationFirJsBoxTestGenerated.java

@@ -0,0 +1,63 @@
+/*
+ * Copyright 2010-2023 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
+ */
+
+package org.jetbrains.kotlinx.serialization.runners;
+
+import com.intellij.testFramework.TestDataPath;
+import org.jetbrains.kotlin.test.util.KtTestUtil;
+import org.jetbrains.kotlin.test.TargetBackend;
+import org.jetbrains.kotlin.test.TestMetadata;
+import org.junit.jupiter.api.Nested;
+import org.junit.jupiter.api.Test;
+
+import java.io.File;
+import java.util.regex.Pattern;
+
+/** This class is generated by {@link org.jetbrains.kotlinx.serialization.TestGeneratorKt}. DO NOT MODIFY MANUALLY */
+@SuppressWarnings("all")
+@TestMetadata("plugins/kotlinx-serialization/testData/boxIr")
+@TestDataPath("$PROJECT_ROOT")
+public class SerializationFirJsBoxTestGenerated extends AbstractSerializationFirJsBoxTest {
+    @Test
+    public void testAllFilesPresentInBoxIr() throws Exception {
+        KtTestUtil.assertAllTestsPresentByMetadataWithExcluded(this.getClass(), new File("plugins/kotlinx-serialization/testData/boxIr"), Pattern.compile("^(.+)\\.kt$"), null, TargetBackend.JS_IR, true);
+    }
+
+    @Test
+    @TestMetadata("constValInSerialName.kt")
+    public void testConstValInSerialName() throws Exception {
+        runTest("plugins/kotlinx-serialization/testData/boxIr/constValInSerialName.kt");
+    }
+
+    @Test
+    @TestMetadata("delegatedProperty.kt")
+    public void testDelegatedProperty() throws Exception {
+        runTest("plugins/kotlinx-serialization/testData/boxIr/delegatedProperty.kt");
+    }
+
+    @Test
+    @TestMetadata("excludedFromExport.kt")
+    public void testExcludedFromExport() throws Exception {
+        runTest("plugins/kotlinx-serialization/testData/boxIr/excludedFromExport.kt");
+    }
+
+    @Test
+    @TestMetadata("excludedFromFileExport.kt")
+    public void testExcludedFromFileExport() throws Exception {
+        runTest("plugins/kotlinx-serialization/testData/boxIr/excludedFromFileExport.kt");
+    }
+
+    @Test
+    @TestMetadata("serializerFactory.kt")
+    public void testSerializerFactory() throws Exception {
+        runTest("plugins/kotlinx-serialization/testData/boxIr/serializerFactory.kt");
+    }
+
+    @Test
+    @TestMetadata("serializerFactoryInUserDefined.kt")
+    public void testSerializerFactoryInUserDefined() throws Exception {
+        runTest("plugins/kotlinx-serialization/testData/boxIr/serializerFactoryInUserDefined.kt");
+    }
+}

plugins/kotlinx-serialization/tests-gen/org/jetbrains/kotlinx/serialization/runners/SerializationFirLightTreeBlackBoxTestGenerated.java

@@ -231,6 +231,18 @@ public class SerializationFirLightTreeBlackBoxTestGenerated extends AbstractSeri
             runTest("plugins/kotlinx-serialization/testData/boxIr/serializableOnPropertyType.kt");
         }
 
+        @Test
+        @TestMetadata("serializerFactory.kt")
+        public void testSerializerFactory() throws Exception {
+            runTest("plugins/kotlinx-serialization/testData/boxIr/serializerFactory.kt");
+        }
+
+        @Test
+        @TestMetadata("serializerFactoryInUserDefined.kt")
+        public void testSerializerFactoryInUserDefined() throws Exception {
+            runTest("plugins/kotlinx-serialization/testData/boxIr/serializerFactoryInUserDefined.kt");
+        }
+
         @Test
         @TestMetadata("starProjections.kt")
         public void testStarProjections() throws Exception {

plugins/kotlinx-serialization/tests-gen/org/jetbrains/kotlinx/serialization/runners/SerializationIrBoxTestGenerated.java

@@ -229,6 +229,18 @@ public class SerializationIrBoxTestGenerated extends AbstractSerializationIrBoxT
         runTest("plugins/kotlinx-serialization/testData/boxIr/serializableOnPropertyType.kt");
     }
 
+    @Test
+    @TestMetadata("serializerFactory.kt")
+    public void testSerializerFactory() throws Exception {
+        runTest("plugins/kotlinx-serialization/testData/boxIr/serializerFactory.kt");
+    }
+
+    @Test
+    @TestMetadata("serializerFactoryInUserDefined.kt")
+    public void testSerializerFactoryInUserDefined() throws Exception {
+        runTest("plugins/kotlinx-serialization/testData/boxIr/serializerFactoryInUserDefined.kt");
+    }
+
     @Test
     @TestMetadata("starProjections.kt")
     public void testStarProjections() throws Exception {

plugins/kotlinx-serialization/tests-gen/org/jetbrains/kotlinx/serialization/runners/SerializationIrJsBoxTestGenerated.java

@@ -48,4 +48,16 @@ public class SerializationIrJsBoxTestGenerated extends AbstractSerializationIrJs
     public void testExcludedFromFileExport() throws Exception {
         runTest("plugins/kotlinx-serialization/testData/boxIr/excludedFromFileExport.kt");
     }
+
+    @Test
+    @TestMetadata("serializerFactory.kt")
+    public void testSerializerFactory() throws Exception {
+        runTest("plugins/kotlinx-serialization/testData/boxIr/serializerFactory.kt");
+    }
+
+    @Test
+    @TestMetadata("serializerFactoryInUserDefined.kt")
+    public void testSerializerFactoryInUserDefined() throws Exception {
+        runTest("plugins/kotlinx-serialization/testData/boxIr/serializerFactoryInUserDefined.kt");
+    }
 }