Add public API functions for internal property management (#3128)

These 4 new API functions give possibility to perform [[Get]], [[Set]], [[Has]], [[Delete]] operations for properties
which are not accessible from the JavaScript context only from the public API.

JerryScript-DCO-1.0-Signed-off-by: Robert Fancsik frobert@inf.u-szeged.hu

jerry-core/lit/lit-magic-strings.h

@@ -48,6 +48,7 @@ typedef enum
   LIT_INTERNAL_MAGIC_STRING_PROMISE_PROPERTY_REMAINING_ELEMENT, /**< [[RemainingElement]] property */
   LIT_INTERNAL_MAGIC_STRING_ITERATOR_NEXT_INDEX, /**< [[%Iterator%NextIndex]] property */
   LIT_INTERNAL_MAGIC_STRING_MAP_KEY, /**< Property key used when an object is a key in a map object */
+  LIT_INTERNAL_MAGIC_API_INTERNAL, /**< Property key used to add non-visible JS properties from the public API  */
   /* List of well known symbols */
   LIT_GLOBAL_SYMBOL_HAS_INSTANCE, /**< @@hasInstance well known symbol */
   LIT_GLOBAL_SYMBOL_IS_CONCAT_SPREADABLE, /**< @@isConcatSpreadable well known symbol */