Option to pair Tethers

[Torgen]

Kel and Ilya are thematically and mechanically linked. Both versions of each have cards that either refer to the other by name, or synergize with what they do in particular. If an expedition has one and not the other, that player is at a significant disadvantage. Give an option for an expedition to not allow one to be in the game if the other isn't.

While Kel and Ilya are the only tethers now, it's possible a future expansion will have another pair, so I'd represent this as a tether?: string field on each mage. It could also be string[] just in case.

I wouldn't force the Kel and Ilya to come from the same expansion, since the one from Legacy of Gravehold has the card that lets them trade cards, so they can arrange to have one give discards and the other have the gem that benefits from being discarded.