The formatter should:
* if the URI is the special "no unit" URI, show no unit.
* if the URI refers to a local item, show the item's label (in the target language, with fallback applied)
** showing a "unit symbol" based on a statement is left for later (T77983)
* otherwise, just show the unit URI
TBD: 3rd party clients must have an efficient way to query and use unit labels.