Two New Guns and Buckshot

Created by: Shadowmech88

Adds a couple of new guns for summon guns/adminbus/putting in vaults/et cetera.

The mahoguny, which fires a single slow-moving projectile that can bounce once and deals 30 damage, along with three fast-moving projectiles in a spread that deal 10 damage each. It can be loaded with wood planks, and holds 10 shots.

The siren, which fires globs of liquid that splash their contents onto what they hit as well as the turf of the thing they hit. The gun has two modes, it can either deal 30 damage per shot and deliver 10u of reagents to the target and its turf, or deal no damage but deliver 50u of reagents to the target and its turf. Its shots will adopt the color of the reagents they contain. Both the gun and its projectiles have the NOREACT flag, which allows for all kinds of on-hit chemical reactions. It can be loaded with any reagents, and holds 5 shots.

Also adds buckshot shells, a box of which can be ordered from Cargo in the shotgun shell crate. A single buckshot shell fires nine buckshot pellets in a small spread, each dealing 10 damage. The shotgun shell crate itself now contains a box of each type of shell rather than 10 loose lethal shells.

Also adds a unique in-hand sprite for sawn-off shotguns, since until now they looked the same as regular shotguns when held. Also makes the blunderbuss and railgun belt-equippable like I've been meaning to do.

The mahoguny and siren ideas and sprites are from Enter the Gungeon, not my own creation.

Addresses #8394.