Support let identifier in non-strict mode. (#3427)

This code adds a lot of checks and complexity to the code.

JerryScript-DCO-1.0-Signed-off-by: Zoltan Herczeg zherczeg.u-szeged@partner.samsung.com

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

@@ -46,6 +46,7 @@ typedef enum
   SCANNER_TYPE_CASE, /**< case statement */
 #if ENABLED (JERRY_ES2015)
   SCANNER_TYPE_INITIALIZER, /**< destructuring binding or assignment pattern with initializer */
+  SCANNER_TYPE_LET_EXPRESSION, /**< let expression */
   SCANNER_TYPE_ERR_REDECLARED, /**< syntax error: a variable is redeclared */
 #endif /* ENABLED (JERRY_ES2015) */
 } scanner_info_type_t;