Refactored the machinery operable procs (#19274)

Refactored the machinery operable procs into a single one, DMDoc'd, SDMM
marked, made more readable.
Fixed suspension field generator not being able to be used as it was not
checking the power cell for operability.

Fixes #19249

code/modules/power/fusion/fuel_assembly/fuel_injector.dm

@@ -29,7 +29,7 @@
 
 /obj/machinery/fusion_fuel_injector/process()
 	if(injecting)
-		if(inoperable())
+		if(!operable())
 			StopInjecting()
 		else
 			Inject()

code/modules/power/fusion/kinetic_harvester.dm

@@ -95,7 +95,7 @@
 			harvesting.Cut()
 
 /obj/machinery/kinetic_harvester/update_icon()
-	if(inoperable())
+	if(!operable())
 		icon_state = "broken"
 	else if(use_power >= POWER_USE_ACTIVE)
 		icon_state = "on"