Basic mobs don't become dense upon death (#72554)

## About The Pull Request

In #72260 what was previously a var became a flag, which was a sensible
change, however this inverted the default behaviour.
In virtually all cases we want dead mobs to _stop_ being dense, this
added a requirement for the flag to be present for that to happen and
then didn't add the flag to any mobs.

Rather than add this to every mob I inverted the function of the flag.
My reasoning here is that _simple_ mobs seemingly never required this
behaviour, basic mobs are probably going to need it rarely if ever, and
including it in `basic_mob_flags` by default seems messy and easy to
leave off when setting other flags (plus #72524 implies to me we want to
avoid adding more default values).

Setting this manually on each mob seems kind of silly as a requirement
going forward and I can't think of a way we'd unit test for people
forgetting.

For the same reason I did the same thing with the
`STOP_ACTING_WHILE_DEAD` flag I added to the AI controller in a recent
PR, the flag should denote unusual behaviour not the default.

## Why It's Good For The Game

It looks really odd when you're constantly shuffling places with dead
mobs, they're not supposed to do that.
It's tedious to add `STOP_ACTING_WHILE_DEAD` to every AI controller when
that should be an obvious default assumption.

## Changelog

🆑
fix: Dead basic mobs are no longer "dense" objects and can be stepped
on.
/🆑

code/modules/mob/living/basic/basic.dm

@@ -149,7 +149,7 @@
 	icon_state = icon_dead
 	if(basic_mob_flags & FLIP_ON_DEATH)
 		transform = transform.Turn(180)
-	if(basic_mob_flags & UNDENSIFY_ON_DEATH)
+	if(!(basic_mob_flags & REMAIN_DENSE_WHILE_DEAD))
 		set_density(FALSE)
 
 /mob/living/basic/revive(full_heal_flags = NONE, excess_healing = 0, force_grab_ghost = FALSE)
@@ -163,7 +163,7 @@
 	icon_state = icon_living
 	if(basic_mob_flags & FLIP_ON_DEATH)
 		transform = transform.Turn(180)
-	if(basic_mob_flags & UNDENSIFY_ON_DEATH)
+	if(!(basic_mob_flags & REMAIN_DENSE_WHILE_DEAD))
 		set_density(initial(density))
 
 /mob/living/basic/proc/melee_attack(atom/target, list/modifiers)