Add `$self` if external references are supported.

[handrews]

PR #296 adds same-document references while raising the possibility of future support for external references. If such support is added, a $self field analogous to the one in OAS 3.2 and the one being added to Arazzo 1.1 should be added to the Overlay specification so that all referencing is consistent across our specification ecosystem.

[handrews]

The manifest idea in issue #291 could also be a use case for Overlay having its own $self, assuming URLs/URIs are used to locate/identify things in the manifest. In particular, if a manifest uses a $self-respecting (hah!) URI for OAD and Arazzo documents, then we really should support such usage with Overlay documents as well.