SHACL: ensure consistency with GeoJSON vocabulary

[hammar]

Presently https://geojson.org/Polygon (defined only in REC, though under a geojson.org namespace) is incompatible with the canonical https://purl.org/geojson/vocab#Polygon

The former is a datatype declaration intended to express that a certain literal (string) resource is a GeoJSON-formatted coordinate listing compliant with the requirements of a GeoJSON polygon.

The latter is a full-fledged class, which itself has a coordinate listing (the syntax and semantics of which are quite clear in the GeoJSON spec, but at the time of writing not in the GeoJSON ontology at the vocab URL given above).

To reduce potential confusion for our users we ought to either find a better datatype to reuse from the canonical GeoJSON ontology, or to define a datatype within our own namespace rather than under geojson.org.

Thanks @ektrah who brought this up.