SSthrowing (#19421)

Ported SSThrowing from TG, to handle throwings.
Updated movement system to the latest iteration, made it a datum as per
latest iteration.
Updated pass/hit handling of atoms, introduced pass_flag_self to
determine what atoms allow to pass.
Moved procs and defines around to make them more organized.

code/game/objects/structures/railing.dm

@@ -8,6 +8,7 @@
 	climbable = TRUE
 	layer = OBJ_LAYER
 	anchored = FALSE
+	pass_flags_self = LETPASSTHROW|PASSSTRUCTURE
 
 	atom_flags = ATOM_FLAG_CHECKS_BORDER
 	obj_flags = OBJ_FLAG_ROTATABLE|OBJ_FLAG_MOVES_UNSUPPORTED
@@ -89,7 +90,7 @@
 /obj/structure/railing/CanPass(atom/movable/mover, turf/target, height=0, air_group=0)
 	if(istype(mover,/obj/item/projectile))
 		return TRUE
-	if(!istype(mover) || mover.checkpass(PASSRAILING))
+	if(!istype(mover) || mover.pass_flags & PASSRAILING)
 		return TRUE
 	if(mover.throwing)
 		return TRUE