Functional M5 Gasmask 2: Electric Boogaloo (#5083)

# About the pull request
Makes the M5 Gasmask actually work
70% NVG code 30% trial and error

### This time I *probably* won't fuck things up.
#5072 
# Explain why it's good for the game
Consistency, the regular gasmask protects you against CN20 Nerve Gas,
whereas the helmet gasmask previously did not.
# Testing Photographs and Procedure
<details>
<summary>Screenshots & Videos</summary>


https://github.com/cmss13-devs/cmss13/assets/95509996/8f4c2eed-4ad6-477d-83d4-1b87630d7ff7

</details>

# Changelog
:cl: Ediblebomb
add: M5 Helmet Gasmask now functions as a gasmask when in a marine
helmet (and yes, gasmasks do still have some functionality)
/:cl:

---------

Co-authored-by: Drathek <[email protected]>

code/game/objects/items/props/helmetgarb.dm

@@ -496,6 +496,30 @@
 	desc = "The USCM had its funding pulled for these when it became apparent that not every deployed enlisted was wearing a helmet 24/7; much to the bafflement of UA High Command."
 	icon_state = "helmet_gasmask"
 
+/obj/item/prop/helmetgarb/helmet_gasmask/on_enter_storage(obj/item/storage/internal/helmet_internal_inventory)
+	..()
+	if(!istype(helmet_internal_inventory))
+		return
+	var/obj/item/clothing/head/helmet/helmet_item = helmet_internal_inventory.master_object
+
+	if(!istype(helmet_item))
+		return
+
+	helmet_item.flags_inventory |= BLOCKGASEFFECT
+	helmet_item.flags_inv_hide |= HIDEFACE
+
+/obj/item/prop/helmetgarb/helmet_gasmask/on_exit_storage(obj/item/storage/internal/helmet_internal_inventory)
+	..()
+	if(!istype(helmet_internal_inventory))
+		return
+	var/obj/item/clothing/head/helmet/helmet_item = helmet_internal_inventory.master_object
+
+	if(!istype(helmet_item))
+		return
+
+	helmet_item.flags_inventory &= ~(BLOCKGASEFFECT)
+	helmet_item.flags_inv_hide &= ~(HIDEFACE)
+
 /obj/item/prop/helmetgarb/trimmed_wire
 	name = "trimmed barbed wire"
 	desc = "It is a length of barbed wire that's had most of the sharp points filed down so that it is safe to handle."