extensibility of Linked-tag-map

[thomas-fossati]

Discuss extensibility of linked-tag-map

Previous discussion captured here:

@nedmsmith: "linked-tag-map is not extensible today and should remain so until a use case is defined that motivates. It is however extensible by $tag-id-type-choice (which aligns with tag-identity-map). These should remain consistently defined."

@thomas-fossati: "I have a slightly different take on this. It doesn't seem implausible -- well, it's actually quite natural -- that one might want to add target attributes to this. In fact, I think we should add it upfront and lock the type as Ned suggests."