Implement ES2015 class feature (part I.) (#2404)

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

Currently supported:
 - Class statement
 - Class expression
 - Static methods

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

jerry-core/parser/js/js-lexer.h

@@ -149,6 +149,7 @@ typedef enum
   LEXER_PROPERTY_SETTER,         /**< property setter function */
   LEXER_COMMA_SEP_LIST,          /**< comma separated bracketed expression list */
   LEXER_SCAN_SWITCH,             /**< special value for switch pre-scan */
+  LEXER_CLASS_CONSTRUCTOR,       /**< special value for class constructor method */
 
   /* Future reserved words: these keywords
    * must form a group after all other keywords. */