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-parser-util.c

@@ -1107,6 +1107,10 @@ parser_error_to_string (parser_error_t error) /**< error code */
     {
       return "Lexical declaration cannot appear in a single-statement context.";
     }
+    case PARSER_ERR_LEXICAL_LET_BINDING:
+    {
+      return "Let binding cannot appear in let/const declarations.";
+    }
     case PARSER_ERR_MISSING_ASSIGN_AFTER_CONST:
     {
       return "Value assignment is expected after a const declaration.";