Resolve flammable itemstack regression.

Using a separate ABM for this.  We may want the burn-up effect with
containers later, but it may have to be handled differently in
those cases as the container itself may have flammability
characteristics to take into account.

This SHOULD also resolve the problem with transient states of the
stack node (i.e. wherein it was temporarily empty) burning up,
which caused some things to burn up when they shouldn't.
Particularly egregious was the destruction of hot lode prills when
they dropped into forgefire.

docs/roadmap.txt

@@ -2,9 +2,6 @@
 ROADMAP (SHORT-TERM)
 ------------------------------------------------------------------------
 
-- REGRESSIONS:
-	- Stack node burn-up when inv is flammable
-		- Maybe it makes sense to have another group for this?
 - Portable containers.
 	- A bag that expands when placed, if room.
 	- Inventory slots around central node each holds 1 stack.

mods/nc_items/depends.txt

@@ -1,2 +1,3 @@
 nc_api
-nc_api_craft
+nc_api_craft
+nc_fire?

mods/nc_items/init.lua

@@ -151,3 +151,23 @@ function minetest.item_place(itemstack, placer, pointed_thing, param2)
 	end
 	return itemstack
 end
+
+if nodecore.loaded_mods().nc_fire then
+	nodecore.register_limited_abm({
+		label = "Flammable ItemStacks Ignite",
+		interval = 5,
+		chance = 1,
+		nodenames = {modname .. ":stack"},
+		neighbors = {"group:igniter"},
+		action = function(pos, node)
+			if nodecore.quenched(pos) then return end
+			
+			local def = invdef(pos)
+			local flam = def and def.groups and def.groups.flammable
+			if not flam then return end
+
+			if math_random(1, flam) ~= 1 then return end
+			nodecore.ignite(pos, node)
+		end
+	})
+	end