Add key/ref check to TEI linter

[paul-butcher]

When looking into wellcomecollection/concepts-pipeline#89, I discovered that some Concepts were being erroneously generated because some keyword items had both key and ref, and the transformer was choosing the wrong one.

That problem is fixed with this PR, but there still remains an inconsistency in the use of @ref vs @key. See some discussion here: https://wellcome.slack.com/archives/CS4G8SAG3/p1669723239637559