Adds the can_charge var to energy guns

If set to 0, makes it so the gun cannot be charged in recharges. Set to
1 by default, only the nuclear (AKA advanced) egun and both plasma
cutters have it set to 0. (Nuclear egun was previously unable to be
charged, but used an istype check specific to it.)

code/modules/projectiles/guns/energy.dm

@@ -9,6 +9,7 @@
 	var/cell_type = "/obj/item/weapon/stock_parts/cell"
 	var/projectile_type = "/obj/item/projectile/beam"
 	var/modifystate
+	var/can_charge = 1
 
 	emp_act(severity)
 		power_supply.use(round(power_supply.charge / severity))

code/modules/projectiles/guns/energy/nuclear.dm

@@ -46,6 +46,7 @@
 	var/lightfail = 0
 	var/charge_tick = 0
 	can_flashlight = 0
+	can_charge = 0
 
 	New()
 		..()

code/modules/projectiles/guns/energy/special.dm

@@ -307,6 +307,7 @@ modules/mob/living/carbon/human/life.dm if you die, you will be zoomed out.
 	charge_cost = 250
 	fire_delay = 10
 	icon_override = 'icons/mob/in-hand/guns.dmi'
+	can_charge = 0
 
 /obj/item/weapon/gun/energy/plasmacutter/examine(mob/user)
 	..()