[MDB Ignore] Tests firedoor regions for fire alarms (#91816)

## About The Pull Request

Ensures regions of firedoors (full rooms, sections of hallways, etc)
have at least one fire alarm within, to ensure people can always reset
them.

Mappers can ignore regions with a mapping helper if they purposefully
want an area to lack a fire alarm.

## Why It's Good For The Game

A sore spot about fire alarms is when mappers forget / neglect to put a
fire alarm in a region of fire alarms, leaving people trapped.

So we brainstormed a way to automatically check that all firedoors can
be pulled or reset on either side, *somewhere*. It's not particularly
picky, so if the fire alarm is 50 tiles away, it'll still count

## Changelog

🆑 Melbert
qol: Firelock setups across every map should be more sane now, with
better coverage of fire alarms and fewer firelock boxes of doom.
/🆑

code/game/objects/effects/landmarks/atmospherics_sanity_landmarks.dm

@@ -33,3 +33,10 @@
 /obj/effect/landmark/atmospheric_sanity/ignore_area
 	name = "Atmospheric Sanity Ignore"
 	icon_state = "atmos_sanity_ignore"
+
+/**
+ * Marks a firedoor region as safely ignorable / doesn't need a fire alarm
+ */
+/obj/effect/landmark/firealarm_sanity
+	name = "Fire Alarm Sanity Ignore"
+	icon_state = "firealarm_sanity"