Refactor chasm paths to be more sane (#32701)

In addition, makes chasm handling a component, so it could be reused for
chasm-likes which are not quite chasms.

Also fixes a couple of bugs relating to building on top of chasms
occasionally erasing the chasm.

code/modules/mob/living/simple_animal/hostile/mining_mobs/necropolis_tendril.dm

@@ -97,5 +97,5 @@
 	visible_message("<span class='boldannounce'>The tendril falls inward, the ground around it widening into a yawning chasm!</span>")
 	for(var/turf/T in range(2,src))
 		if(!T.density)
-			T.TerraformTurf(/turf/open/chasm/straight_down/lava_land_surface)
+			T.TerraformTurf(/turf/open/chasm/lavaland, /turf/open/chasm/lavaland)
 	qdel(src)

code/modules/ruins/objects_and_mobs/sin_ruins.dm

@@ -102,7 +102,7 @@
 	"<span class='notice'>Perfect. Much better! Now <i>nobody</i> will be able to resist yo-</span>")
 	var/turf/T = get_turf(user)
 	T.ChangeTurf(/turf/open/chasm/straight_down)
-	var/turf/open/chasm/straight_down/C = T
+	var/turf/open/chasm/C = T
 	C.drop(user)
 
 //can't be bothered to do sloth right now, will make later