Item Specific Crit Details

Oh that critical hit thing is gonna hit MRE too isn't it... hrm...

MR In question:
https://gitlab.com/foundrynet/dnd5e/-/merge_requests/398/diffs

In short, this adds a getCriticalThreshold method on Item5e, which replaces some of the logic in Item5e#rollAttack()

It also changes item.data.critical to be an object (what was it before??) with:
- threshold that appears to be a number similar to the one in actor flags
- damage that seems to be the same shape as a damage part?

I can't figure out what item.data.critical was used for before... I think nothing?

cc @tposney ^