Introduce the `Iri` content type

[vaitkus]

The Internationalized Resource Identifier (IRI, https://datatracker.ietf.org/doc/html/rfc3987) can be viewed as an extension of the Universal Resource Identifier that permits a greater character set (most characters from UCS). As such, all URIs are also IRIs. Given that CIF 2.0 and DDLm also have an extended character set, it might make sense to introduce the IRI content type instead or alongside the URI content type.

This idea was originally sparked by a PR in the cif_core repository (see https://github.com/COMCIFS/cif_core/pull/586/files#r2826402823).

[jamesrhester]

I think adding an IRI type is reasonable. We should keep URI as well.