Adds the Marksman Revolver from Ultrakill, with coinflipping and splitshotting! (#70612)

## About The Pull Request
Ultrakill is a neat game that I like, and I like to add fun things from
things I like. This PR adds the marksman revolver as an admin-only
weapon, complete with coin flipping and splitshotting mechanics! [Have a
video demonstration!](https://streamable.com/5x4mnj)

Basically, right clicking on a visible tile flips a coin towards a
random adjacent tile to the one you targeted. Shooting the coin
projectile with the hitscan primary fire while it's within one tile of
the target will trigger a splitshot. Those two child splitshots will
roughly follow the priority order of Ultrakill (prioritizing coins [for
even more splits!], then nearby mobs, then less nearby mobs, then other
random stuff nearby). I plan to add other stuff for the shots to
prioritize like fuel tanks, windows, and others.

I'm still working out some of the finer details of balance and more
closely matching how it works to UK (while still working in the realm of
SS13), and of course I hope to get unique sprites/sounds/whatever.

<img src='https://imgur.com/XUTqksD.png'>

## Why It's Good For The Game
Adds more funny/unique things for admins to spawn in.

## Changelog

🆑 Ryll/Shaps
add: Added the Marksman Revolver, for admins to spawn.
/🆑

code/game/objects/effects/temporary_visuals/projectiles/muzzle.dm

@@ -38,3 +38,6 @@
 
 /obj/effect/projectile/muzzle/sniper
 	icon_state = "sniper"
+
+/obj/effect/projectile/muzzle/bullet
+	icon_state = "muzzle_bullet"

code/game/objects/effects/temporary_visuals/projectiles/tracer.dm

@@ -59,6 +59,12 @@
 	name = "solar beam"
 	icon_state = "solar"
 
+/obj/effect/projectile/tracer/solar/thin
+	icon_state = "solar_thin"
+
+/obj/effect/projectile/tracer/solar/thinnest
+	icon_state = "solar_thinnest"
+
 //BEAM RIFLE
 /obj/effect/projectile/tracer/tracer/beam_rifle
 	icon_state = "tracer_beam"