Delve and taxing effects

When a taxing effect like thorn of amethyst is in play I should still be able to cast my delve spells by simply exiling and extra card from the graveyard (if available).

Start by locating the implementation of delve spell costs and the taxing-effect rules, then reproduce the interaction with Thorn of Amethyst and a delve spell. Verify whether an additional graveyard card can be exiled to pay the tax and add or update a regression test for this scenario. The issue names no files or existing tests.

Written by the indexing model from the issue text.