Egg Changes (#5054)

# About the pull request
Now that #4716 's been merged,
here's the changes I've wanted to make for eggs for a bit now.

- Eggs can no longer be planted in vehicles
- Eggs can no longer be planted on tiles that would obstruct the egg. 

# Explain why it's good for the game

- In the vast majority of cases, you cannot tell that a vehicle you're
entering is infested with you-instantly-lose eggs. While normally
getting hit with an egg is reasonably predictable (there are weeds ==
possibility of eggs), having no indication of it until you go inside and
get hugged isn't fun or balanced.
- Eggs being able to layer under things like grass is technically a
"feature", but not one I would like to keep. Traps serve the exact same
purpose, but are much harder to mass produce (as intended). Eggs should
be cheaper but more detectable, which doesn't align them being able to
be near-fully hidden.
# Testing Photographs and Procedure
<details>
<summary>Screenshots & Videos</summary>


![image](https://github.com/cmss13-devs/cmss13/assets/41448081/7151c503-72fa-4b1e-acba-576f556420c1)


</details>


# Changelog
:cl:
balance: You cannot plant eggs inside vehicles.
balance: Eggs can no longer be planted on tiles with objects that would
obscure them.
/:cl:

code/modules/cm_aliens/structures/egg.dm

@@ -8,7 +8,7 @@
 	icon_state = "Egg Growing"
 	density = FALSE
 	anchored = TRUE
-	layer = LYING_BETWEEN_MOB_LAYER //to stop hiding eggs under corpses
+	layer = LYING_BETWEEN_MOB_LAYER
 	health = 80
 	plane = GAME_PLANE
 	var/list/egg_triggers = list()

code/modules/mob/living/carbon/xenomorph/egg_item.dm

@@ -102,6 +102,16 @@
 		to_chat(user, SPAN_XENOWARNING("[src] can only be planted on [lowertext(hive.prefix)]hive weeds."))
 		return
 
+	if(istype(get_area(T), /area/vehicle))
+		to_chat(user, SPAN_XENOWARNING("[src] cannot be planted inside a vehicle."))
+		return
+
+	for(var/obj/object in T.contents)
+		var/obj/effect/alien/egg/xeno_egg = /obj/effect/alien/egg
+		if(object.layer > initial(xeno_egg.layer))
+			to_chat(user, SPAN_XENOWARNING("[src] cannot be planted below objects that would obscure it."))
+			return
+
 	user.visible_message(SPAN_XENONOTICE("[user] starts planting [src]."), SPAN_XENONOTICE("You start planting [src]."), null, 5)
 
 	var/plant_time = 35