Extract terms defined with "data-export" attribute

[tidoust]

Cross-reference links to terms defined in another spec should be limited to term definitions that are explicitly marked up as exported through a data-export attribute, since these terms represent the stable interface that authors are ready to support.

Reffy should extract these terms and their ID, and report an anomaly when a spec references a term that is not in the list of exported terms.

Note spec edition tools already automatically mark WebIDL terms with a data-export="" attribute, so no need to treat them differently.

[dontcallmedom]

This was done in #229