redo the way experience is gained and tracked

[zkat]

We made an assumption when working on IV initially that special tracks would be the source of experience, because that's how it is in Starforged.

In Ironsworn classic, though, experience is gained through miscellaneous things like completing vows or the results of some asset rolls, and the single special track, Bonds, doesn't have anything to do with experience.

We need to discuss a good design for this. One quick way around it is to turn the "XP Gained" field in character sheets into an input, and make it so XP Gained - Starforged Special Track XP = additional_xp in a field in the frontmatter.

Or we could do the most robust thing and just make it so you have to hand-maintain any experience gained, whether in Ironsworn or in Starforged.

Later, we could consider adding the little xp pips under special track boxes for Starforged, like the standard character sheet or Foundry do.