Add a token which returns the index of a key in a data file. This can be used in a few specialized cases like adding an NPC to a festival map.

For example, Abigail is the first entry in Data/NPCDispositions so {{DataFileIndex: Data/NPCDispositions, Abigail}} = 0.

This isn't feasible with the way Content Patcher is implemented; the token needs to load the asset to get its value, and tokens loading assets does weird things. This is only really needed for a specific case, so I'll suggest a change in the game code to accommodate that case instead.