Exosuit Nuclear Power Cores (#18268)

* Added power cores, a type of large battery cell that get used by
exosuits. The nuclear and phoron variants are self-charging.
* Combat mechs now start with nuclear power cores, allowing them to
sustain themselves indefinitely, so long as they stay out of the action
for a bit.
* Removed basic power cells from the mechfab, replaced with the mech
powercores.
* Mech cell statuses now instantly update as soon as the cell charges or
discharges.
* Added a stack of 10 uranium to the machinist's workshop, which can
print two nuclear power cores.

code/modules/heavy_vehicle/mecha.dm

@@ -208,6 +208,8 @@
 		if(source_frame.body)
 			source_frame.body.forceMove(src)
 			body = source_frame.body
+			if(body.cell)
+				RegisterSignal(body.cell, COMSIG_CELL_CHARGE, PROC_REF(handle_cell_charge))
 
 	updatehealth()