Fix 20ga percussion ammo inheritance and names

[chaosvolt]

Summary

SUMMARY: Bugfixes "Fix up abstract used by 20 gauge percussion shells so they don't drop shotgun hulls, standardize naming convention"

Purpose of change

It was pointed out that the cap-and-ball shotshells were leaving behind regular plastic shotshells as casings due to the ammo inheriting from regular 12 gauge shot, when it turns out that trying to delete a casing entry inherited from something else doesn't quite work right.

Along the way it was also found that the ammo has its own abstract entry but it inherits from the regular shot abstract, and moreover was actually only used by the slug variant. And finally, the names were all over the place in a way that made them confusing, easily mistaken for the flintlock cartridges for one.

Describe the solution

1. Properly defined the abstract used by paper shot to be its own thing, so that its stats are clearly defined and it won't inherit shell casings. also moved it to the top of the file for visibility.
2. Set 20ga paper shot to properly inherit from the new abstract instead of blackpowder 00 shot, which means all the other variants will inherit the correct info due to the others inheriting from buckshot.
3. Standardized the names to be more consistent, all of them following the format of specifying they're 20 gauge, along with their general type of loading. .62 also could sorta work but it was pointed out to me that using its designation in shotgun units makes it more clear that this is intended to be a cap and ball shotgun round, whereas .62 would make it more easily confused for a rifle round.

Describe alternatives you've considered

Screaming.

Testing

1. Checked affected file for syntax and lint errors.
2. Load-tested in compiled build, and compared stats to the pre-change version on my desktop save to confirm none of the stats got changed as a consequence.

Additional context

This doesn't matter much right now since the lemat's secondary is the only thing that uses it, but @hirohito1 has plans to add a percussion-cap shotgun that will also use it, on the basis that percussion muzzleloaders are way more common a hunting weapon than flintlocks, meaning there's a valid niche that this can reasonably fit into.

If one of us also makes reproduction loads that use a small amount of smokeless powder a thing it'll get even more usable, since players would be able to take advantage of powder scavenged from unused modern ammo they find. Much like how the makeshift overpressure rounds for .36 and .44 in Cataclysm++ make for a massive boost in how usable the Colt Navy and Army are.