limit oneway arrows to currently supported highway features

[claysmalley]

Currently, oneway arrows appear on railroads in Americana. oneway=yes on railway=* is a tagging error mistakenly exposed on OpenMapTiles, and railway:preferred_direction=forward|backward is the correct tag for a track on which trains only travel in one direction in most circumstances. This PR limits oneway arrows to highway features, preventing them from showing up on railroads.

production, preview, localhost

before:

after:

[ZeLonewolf]

Is there a corresponding ticket in OMT that should be opened?

[ZeLonewolf]

Looks good. I agree with being explicit about the classes with oneway treatment. Once we have finer control over the tile schema, we might de-optimize that check and handle this on the tile side. The layer size increase is not significant (~400 bytes).