Adds Lawyer/HoP/QM/Spy unique traitor item, the briefcase firearm mechanism (#95607)

## About The Pull Request

Lawyers, Quartermasters, and Heads of Personnels can now purchase a
`"Briefcase Embedded Firearm Trigger"` from the traitor uplink for 4 tc.

The briefcase looks like a normal, unassuming briefcase - But when any
weapon is placed inside, you gain the ability to "fire" the briefcase,
which *actually* fires the weapon.

Only the first weapon found is fired, even if it's empty.
So while you CAN put 5 pistols inside, you'd have to cycle them in and
out manually.

This works with *any* weapon that can fit inside the briefcase, which
essentially restricts it to pistols and some laser weapons.
*No*, sniper rifles won't work in the briefcase. (You can't use it to
bypass weapons that require two hands.)
*Yes*, you can also handcuff the briefcase to your wrist as normal. (Who
knows what this will allow.)

---

Also purchaseable is an 8tc `"Briefcase Embedded Firearm (Combo Deal)"`,
which comes pre-loaded with a Makarov and 0 magazines.

This variant is also available to all Spies as a medium difficulty
reward.

--- 

Additionally I changed the boot-dagger's examine tell to only show to
people holding the item.
When I was implementing the briefcase gun I made the examine tell only
show while held and I thought "well, the boot-dagger should follow the
same logic". They're meant to be stealth objects so being able to figure
it out from across the room feels weird.

## Why It's Good For The Game

This was a random idea someone threw out that I thought fit the vibe and
gameplay of traitors and spies really well.
It's a very espionage-y idea, very James Bond. 

It will definitely catch a lot of people off guard. It basically allows
the Lawyer to walk around with a pistol or revolver drawn at all times.
I'm a bit worried it'll be too potent combined with handcuffs (nodrop
revolver?), but the Lawyer doesn't get many Ws so maybe it's fine.

## Changelog

🆑 Melbert
add: Traitor Lawyers, Quartermasters, and Heads of Personnels can now
purchase a "Briefcase Embedded Firearm Trigger" from the uplink for 4tc
- a briefcase that allows any weapon stored within to be fired by
"firing" the briefcase itself. Weapon sizes are restricted to whatever
fits in a briefcase, meaning it practically only works with pistols and
some laser weapons. It has a subtle examine tell that only appears while
being held.
add: Also available for 8tc is a combo deal that comes pre-loaded with a
Makarov. This can also appear as a medium Spy reward.
balance: The Spy item "boot dagger"'s examine tell now only appears if
you're examining them WHILE holding them.
/🆑

code/datums/storage/subtypes/others/misc.dm

@@ -144,6 +144,26 @@
 /datum/storage/briefcase
 	max_total_storage = 21
 
+/datum/storage/briefcase/gun
+	max_slots = 5
+	max_total_storage = 15
+	/// Max weapons weight that can be stored within, inclusive
+	var/max_weapon_weight = WEAPON_MEDIUM
+
+/datum/storage/briefcase/gun/can_insert(obj/item/to_insert, mob/user, messages, force)
+	. = ..()
+	if(!.)
+		return FALSE
+
+	if(istype(to_insert, /obj/item/gun))
+		var/obj/item/gun/gun = to_insert
+		if(gun.weapon_weight > max_weapon_weight)
+			if(messages && user)
+				user.balloon_alert(user, "too heavy!")
+			return FALSE
+
+	return TRUE
+
 ///Pill bottle
 /datum/storage/pillbottle
 	allow_quick_gather = TRUE