Various bullet fixes and changes (#22503)

fixes https://github.com/Aurorastation/Aurora.3/issues/22370 and
https://github.com/Aurorastation/Aurora.3/issues/22475

I decided to lower the amount of bullets to increase the item pile, as
otherwise removing a single shotgun shell from a pile would be enough to
reduce its size, and that felt wrong.

  - rscadd: "Added the ability to stack spent bullet casings."
- balance: "Ammo piles now increase in size the second time when there
is 6 items in the pile, rather than 8"
- bugfix: "Fixed bullet casings being ejected when trying to suicide
with a gun on safety."
- bugfix: "Fixed that bullet piles didn't update their size correctly."
- bugfix: "Fixed that shotgun practice shells were labelled incorrectly
in the autolathe."
  - bugfix: "Singular bullets no longer layer under tables."

code/modules/fabrication/designs/general/designs_ammo.dm

@@ -50,7 +50,7 @@ ABSTRACT_TYPE(/singleton/fabricator_recipe/ammunition)
 	security_level = SEC_LEVEL_RED
 
 /singleton/fabricator_recipe/ammunition/shotgun/practice
-	name = "shells (buckshot, shotgun)"
+	name = "shells (practice, shotgun)"
 	path = /obj/item/ammo_pile/shotgun_practice
 
 // Revolvers