Add `hgv` to access field

[arch0345]

Ideally this should only be visible for highway types where motor vehicles are allowed (ie not footways). Adding hgv would make it easier for mappers to indicate whether or not trucks are allowed and whether through truck traffic is allowed.

Truck access signage is relatively common and can be mapped by looking at street-level imagery. For example:

hgv=destination

hgv=no

Other examples can be found on the wiki: https://wiki.openstreetmap.org/wiki/Key:hgv#Tagging_Examples

[1ec5]

Previously openstreetmap/iD#3946.

[tyrasd]

also related: https://github.com/openstreetmap/iD/issues/3945 and #378

[RudyTheDev]

Anecdotally, consider that hgv is used 770K times and horse is used 1.5M times. I have to wonder how much of it is due to iD having the latter field, but not the former (and also it being a weird acronym). Given that access is de jure for legal restriction but a de facto mix of legal and intended/implied/possible use. Personally, I know of a single location in my area that has an actual legal horse=no traffic signed area, and I have not seen a single other use of horse that was tagged for any other reason than "I think horses can('t) go here". Yet every tenth road^{[citation needed]} has a "no hgv" sign -- these are basically all over the place. If anything, I would get rid of the horse field than not have an hgv field. Ideally though, horse should be only shown in countries/regions where it's actually commonplace to have traffic signs for horses and bridleways and such, like UK.

Basically, this sign is everywhere in Europe:

[AntMadeira]

This would be a great addition. And imho it even could substitute the horse access field.