RULES - add definitions for all supported rules (#375)
RULES - add definitions for all supported rules (#375)

* RULES - add definitions for all supported error rules

* RULES - add definitions for all supported warning rules

* Apply suggestions from code review

Co-authored-by: Lionel Nébot Janvier <5befb0748e190886505727a102475cc24fbaa116@mobilitydata.org>
MobilityData/gtfs-validator,MobilityData/gtfs-validator,MobilityData/gtfs-validator,MobilityData/gtfs-validator