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)