CDRIVER-2208: bson_as_canonical_json() and bson_as_relaxed_json()

Introduce new functions for the canonical and relaxed formats from Extended
JSON Spec 2.0. This reverts previous changes to bson_as_json(), which might
have caused BC breaks. Maintain legacy bson_as_json() output for non-finite
values. Note that legacy mode will produce "nan" and "inf" literals, which
are not valid JSON.

Also adds helper function for appending sorted regex options and regression
tests for bson_as_json() and multiple types.