Add support for parsing of REF_TO keyword (defined in IEC61131-3 v3). This support is currently inactive (in flex) until we get to add C code generation later on.

STD_TEST.st
STD_RESSOURCE.c
LOCATED_VARIABLES.h
POUS.c
test
STD_CONF.c
SFC_TEST.st
test.exe
plc.st
POUS.h
VARIABLES.csv