Add road vehicle height restriction units

[1ec5]

Added a new feature property to indicate the unit of length posted on vehicle height restrictions. This information will allow iD’s Maximum Height field order to default to the locally appropriate unit: openstreetmap/iD#3614.

According to CLDR, Canada, India, the United Kingdom, and the United States use feet and inches for human-scale height measurements, while only the United Kingdom and United States measure road distances in miles instead of meters. That would imply that the U.K. and U.S. alone would measure vehicle height restrictions in feet and inches. However, not all U.S. overseas dependencies measure height in feet and inches (Puerto Rico being a notable exception), and various British overseas territories are represented separately in this JSON file. I did my best to figure out the height units in each territory based on this Wikipedia article, but it’s entirely possible that I under- or overcorrected.

This PR also corrects the road speed unit in some regions: Akrotiri and Dhekelia apparently uses the same signs as the Republic of Cyprus, which expresses speed limits in kilometers per hour. Some additional British overseas territories (Anguilla, South Georgia and South Sandwich Islands, and Montserrat) use imperial units, according to this table.

[1ec5]

I’m open to calling this property something more generic. It’s pretty likely that vehicle lengths and distances between vehicles are expressed in the same unit as vehicle heights in every jurisdiction. On the other hand, iD may eventually need a separate property for human height measurements, which as stated above are expressed in feet and inches in Canada, India, and possibly elsewhere.

[quincylvania]

@1ec5 Thanks! This is a great addition 👍

I’m open to calling this property something more generic.

I think it's fine as-is, we don't need to guarantee that these units will also be valid for other values.