Currently the extended JSON writer is directly encoded into the ValueWriter for extended JSON. This means that the ValueWriter API must be used to write extended JSON and that any packages that bsonrw depends on cannot write extended JSON. Since the bsonrw package depends on the bsoncore package this means the bsoncore has to have it's own implementation of this generic logic. Separate this common logic into it's own (potentially internal) package.

Specific targets:

• bson.RawElement.String and bsoncore.Element.String have totally different Extended JSON marshaling logic.