mortar shells will now cook off and explode when flamed

[iloveloopers]

About the pull request

title

Explain why it's good for the game

immersion. ammo boxes do it, why not shells?
realism (probably)
its gonna be real funny on evac

Testing Photographs and Procedure

shell_fire.mp4

Changelog

🆑
add: Mortar shells will now cook off and explode when flamed
/:cl:

[vero5123]

You got a runtime buddy.

[2024-05-11 16:40:20.791] Starting up round ID 1
 - -------------------------)
[2024-05-11 16:40:20.814] Running CMSS13 revision: 2024-05-05
 - origin/master: 772d2f31ef35782affc8a6f2be553ca03d864c40
 - HEAD: 22e35ca812931d93912f23e9fb0ac222619f5999
[2024-05-11 17:24:17.210] runtime error: cell_explosion called without cause_data.
 - proc name: stack trace (/proc/stack_trace)
 -   source file: code/__HELPERS/unsorted.dm,1834
 -   usr: (src)
 -   src: null
 -   call stack:
 - stack trace("cell_explosion called without ...")
 - cell explosion(the plating (170,64,3) (/turf/open/floor/plating/almayer), 60, 20, 0, null, null)
 - the 80mm fragmentation mortar ... (/obj/item/mortar_shell/frag): detonate(the plating (170,64,3) (/turf/open/floor/plating/almayer))
 - /datum/callback (/datum/callback): InvokeAsync()
 - 

[iloveloopers]

Man i can never get a perfect one can i?