KJS: turn Json class into external interface

#KT-12712 Fixed

js/js.libraries/src/core/json.kt

@@ -1,6 +1,6 @@
 package kotlin.js
 
-public external class Json() {
+public external interface Json {
     operator fun get(propertyName: String): Any?
     operator fun set(propertyName: String, value: Any?): Unit
 }