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/diagnostics/companionObjectSerializers.kt

@@ -0,0 +1,39 @@
+// FIR_IDENTICAL
+// SKIP_TXT
+
+// FILE: test.kt
+import kotlinx.serialization.*
+
+class Bar
+
+<!COMPANION_OBJECT_AS_CUSTOM_SERIALIZER_DEPRECATED!>@Serializable<!>
+class Foo1 {
+    @Serializer(Foo1::class)
+    companion object
+}
+
+@Serializable
+class Foo2 {
+    <!COMPANION_OBJECT_SERIALIZER_INSIDE_OTHER_SERIALIZABLE_CLASS!>@Serializer(Bar::class)<!>
+    companion object
+}
+
+@Serializer(Foo3::class)
+object Foo3Ser
+
+@Serializable(Foo3Ser::class)
+class Foo3 {
+    <!COMPANION_OBJECT_SERIALIZER_INSIDE_OTHER_SERIALIZABLE_CLASS!>@Serializer(Bar::class)<!>
+    companion object
+}
+
+@Serializable(Foo4.Companion::class)
+class Foo4 {
+    @Serializer(Foo4::class)
+    companion object
+}
+
+class NonSerializableFoo {
+    <!COMPANION_OBJECT_SERIALIZER_INSIDE_NON_SERIALIZABLE_CLASS!>@Serializer(Bar::class)<!>
+    companion object
+}