Change: add constants for FRAX cargo classes

[andythenorth]

~12 years of current cargo classes shows that some are very useful and some are not.

FRAX clarifies or redefines a number of cargo classes, and will be supported in FIRS and Iron Horse. https://grf.farm/polar_fox/frax_latest.html

Providing constants matching FRAX reduces friction for grf authors. Alternately I can provide a constants file for them to import to their nml, but that's the same result for more friction :)