Add checker for companion objects inside serializable class:

Warning for old FE, error in FIR

#KT-54441 Fixed

Merged-by: Leonid Startsev <leonid.startsev@jetbrains.com>

plugins/kotlinx-serialization/testData/firMembers/serializerViaCompanion.fir.txt

@@ -1,4 +1,5 @@
 FILE: serializerViaCompanion.kt
+    @FILE:R|kotlin/Suppress|(names = vararg(String(COMPANION_OBJECT_AS_CUSTOM_SERIALIZER_DEPRECATED)))
     package com.example
 
     @R|kotlinx/serialization/Serializable|() public final data class WithCompanion : R|kotlin/Any| {

plugins/kotlinx-serialization/testData/firMembers/serializerViaCompanion.kt

@@ -1,5 +1,7 @@
 // WITH_STDLIB
+// !DIAGNOSTICS: -COMPANION_OBJECT_AS_CUSTOM_SERIALIZER_DEPRECATED
 
+@file:Suppress("COMPANION_OBJECT_AS_CUSTOM_SERIALIZER_DEPRECATED")
 package com.example
 
 import kotlinx.serialization.*