Add highway=mini_roundabout to a future version of the schema

[SomeoneElseOSM]

Currently at https://taginfo.openstreetmap.org/keys/highway#values there are 58k of these worldwide, so it's a frequently used value. Usage is spread evenly across the world; it's not a value that is used more commonly by some OSM communities to express that concept than others. It's documented at https://wiki.openstreetmap.org/wiki/Tag%3Ahighway%3Dmini_roundabout .

It would add value to a general purpose map style that may want to use the Shortbread schema.

Where it should go is a bit of an open question. There is https://shortbread-tiles.org/schema/1.1/#layer-street_labels_points , which currently just has motorway exit labels in it. A new "street_points" layer might make sense, or in https://shortbread-tiles.org/schema/1.1/#layer-pois along with "highway=emergency_access_point". If the latter was chosen, it'd be unlikely that it would affect existing clients.