This PR adds a layer for `highway=motorway_junction` features. This implementation of highway exits in the transportation_name layer add to the existing layer table structure, and renames the internal column name from "construction" (which was already overloaded with non-construction usages) to "subclass", which will be less confusing to future developers. The string 'junction' is used as the universal sub-class for highway exits. A new documentation PR has been opened at openmaptiles/www.openmaptiles.org#69 to reflect these changes in the documentation.
transportation
Docs
Read the layer documentation at http://openmaptiles.org/schema#transportation

