New functions:
 - ecma_create_symbol
 - ecma_value_is_symbol
 - jerry_get_symbol_descriptive_string

Also improve the jerryx_handler_print to be able to print symbol values via using the jerry_get_symbol_descriptive_string API function for symbol values.

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