Add weapon mod locations

Current locations:

ammo
auxiliary
barrel
bore
conversion
grip
mechanism
sights
stock

Not currently used for anything. Decided I would code this after a discussion in PR #5101. I have a few ideas on what to do with the locations, but I want to see what everyone else thinks before I just go ahead and implement it my way. The original idea was to restrict certain mods from being installable on crossbows. Any ideas on what to use these for?