Add a percussion cap shotgun

[chaosvolt]

Summary

SUMMARY: Content "Add a percussion cap shotgun to represent blackpowder hunting spawns and add more use of the underutilized 20 ga percussion ammo"

Purpose of change

This adds a cap and ball firearm that should reasonably fit into a viable spawn niche a bit more easily than other blackpowder guns, on the basis that percussion cap rifles/shotguns are the norm for modern blackpowder hunting rather than flintlocks. Cap and ball ammo also is interesting since it fits in between flintlock and regular ammo in terms of how easy it is to , and this adds some use for percussion cap shotshells since it's otherwise only use by the Lemat's secondary, which is anan extremely obscure use.

An idea @hirohito1 (EDIT: hecc, wrong username at first) and I messed around with, they've got an idea for a more survivor-esque firearm in this vein while I ended up working on the more vanilla-flavored take on this, they're busy with IRL stuff so I wound up tackling the idea for the basic percussion shotgun. Also pairs well with #3195

Describe the solution

1. Added a 20 gauge percussion muzzleloading shotgun, flavored as a likely modern reproduction for sporting use. Currently generic instead of branded, like the flintlocks and break-action shotguns. Stats lean roughly in between the single-barrel shotgun and flintlock musket. Damage and range are neutral since the percussion cap ammo is balanced about right as it is, and the Lemat shotty has penalties to that so longer barrel fits that. Reload rate is slower than any of the other (pistol-sized, mind) cap and ball guns while a bit faster than most of the flintlocks, it's still a muzzleloader but putting a cap on would be a bit faster than priming a pan.
2. Defined nested_shotgun_paper, everyday_shotgun_paper, and on_hand_shotpaper itemgroups for nested gun spawns.
3. Added nested_shotgun_paper to guns_shotgun_common at same weight as single and double shotgun. Idea being it'd be the most likely blackpowder gun to show up in the "common" itemgroup for storage and display.
4. Added everyday_shotgun_paper to carried_guns_shotgun_common and guns_survival at same rates as single and double shotgun. It might make more sense for it to be common in storage and display but count as obscure for regular Zero Day survivors to reach for, but right guns tend to only stay in one category at a time. Its use for survivalists at least seems more reasonable and fits with how one of these days I want to add itemgroups for rural gun spawns.
5. Added 20 gauge shotgun to guns_shotgun_common_display, same weight as single and double shotgun.
6. Added the standard shot and slug ammo to ammo_shotgun_common at a combined weight matching that of birdshot or slugs, and pyrotechnic variant to ammo_shotgun_rare at half the weight of dragon's breath shells.
7. Added the standard trio to ammo_shotgun_blackpowder_handloads at a combined weight matching blackpowder birdshot.
8. Added the shotgun to antique, antique_rare, antique_bank_vault home_display_case, museum_guns (also some ammo), pawn, antique_guns, antique_guns_with_ammo. Basically groups where blackpowder guns normally spawn on their own.
9. Added the main trio of buck/bird/slug to museum_guns, and antique_ammo.

Describe alternatives you've considered

1. Flavoring it as a specific model of shotgun. Only 20 gauges I could find seem to mainly be made by an Italian brand called Pedersoli, and it seems for percussion cap ones specifically they mainly make side-by-side double barrels rather than single-shot.
2. Splitting the gun spawns between common and obscure depending on whether it's being stored on carried. Something not really done with other gun spawns but might be reasonable for this to be a gun that's reasonably easy to find in stores and in the gun collections of blackpowder hunters, while being something someone wouldn't normally reach for on Zero Day. On the other hand, as mentioned when I get to rural gun spawns I'd likely have to revisit that idea anyway...
3. Bumping the weight of the 20 gauge up in guns_survival, again probably something that'll just end up bein covered should I ever do the rural gun spawn idea.

Testing

1. Checked affected files for syntax and lint errors.
2. Load-tested in compiled build, spawned in the shotgun to play about with it.

Additional context