Implement ES2015 class feature (part II.) (#2439)

This patch is the second milestone of the implementation of this new language element.

Supported:
 - Single class inheritance
 - Functionality of 'super' keyword
 - Implicit constructor in class heritage
 - Specific behaviour while extending with the built-in 'Array' or '%TypedArray%' object
 - Abstract subclasses (Mix-ins)

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

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

@@ -52,6 +52,9 @@ typedef enum
                                                                                  *   deleted properties */
 
   LIT_INTERNAL_MAGIC_STRING_NATIVE_POINTER, /**< native pointer info associated with an object */
+  LIT_FIRST_INTERNAL_MAGIC_STRING = LIT_INTERNAL_MAGIC_STRING_NATIVE_POINTER,  /**< first index of internal
+                                                                                *   magic strings */
+  LIT_INTERNAL_MAGIC_STRING_CLASS_THIS_BINDING, /**< the this binding of the class constructor */
   LIT_MAGIC_STRING__COUNT /**< number of magic strings */
 } lit_magic_string_id_t;