Add field type for key with directional subkeys

[1ec5]

The Cycleway field should be generalized so that some field can set *:forward and *:backward subkeys and perhaps the parent key too. For example, users currently have to use the raw tag editor whenever a road has an odd number of lanes or they’re unevenly distributed. A field similar to Cycleway could allow the user to set Forward, Backward, and Both Ways lane counts right beneath the total number of lanes. The Speed Limit field could allow the user to set different speed limits in the forward and backward directions whenever that happens on the ground.

/ref #387 #6415