Include WasExperimental into builtins serialization

It is required for stabilizing previously experimental
builtin functions and classes (KT-53778)
in order to mark them WasExperimental,
so that they are available for previously opted in users

core/builtins/build.gradle.kts

@@ -44,6 +44,7 @@ val prepareRangeSources by tasks.registering(Sync::class) {
     from(kotlinAnnotationsCommon) {
         include("ExperimentalStdlibApi.kt")
         include("OptIn.kt")
+        include("WasExperimental.kt")
     }
 
     into(rangesCherryPicked)