add helper table mapping extended to basic route_types

[derhuerst]

Working with extended route_type values (both in the Google scheme as well as in the TPEG-PTI scheme) is cumbersome.

For example, most ferries are specified as 4 ("Ferry. Used for short- and long-distance boat service."), while some are specified as 1000 ("Water Transport Service").

gtfs-via-postgres (and gtfs-via-duckdb) should provide a mapping table that maps them to basic route_type values, and maybe also add basic_route_type to arrivals_departures/connections.