Stairs: Add mossy cobble slab and stair

Allow water to turn cobble slab and stairs to turn into mossy versions.

There is no crafting recipe for mossy stairs and mossy slabs, the
stair/slab API has been modified to allow for a recipeitem that
is `nil`, which will omit adding a crafting recipe for these two
items. The API documentation is updated.

The slabs and stairs will turn mossy when water is adjacent, just like
cobblestone. You can either farm mossy versions by placing them in
water for a while, then collecting them, or run water over your craft.

mods/default/functions.lua

@@ -466,12 +466,18 @@ minetest.register_abm({
 --
 
 minetest.register_abm({
-	nodenames = {"default:cobble"},
+	nodenames = {"default:cobble", "stairs:slab_cobble", "stairs:stair_cobble"},
 	neighbors = {"group:water"},
 	interval = 16,
 	chance = 200,
 	catch_up = false,
 	action = function(pos, node)
-		minetest.set_node(pos, {name = "default:mossycobble"})
+		if node.name == "default:cobble" then
+			minetest.set_node(pos, {name = "default:mossycobble"})
+		elseif node.name == "stairs:slab_cobble" then
+			minetest.set_node(pos, {name = "stairs:slab_mossycobble", param2 = node.param2})
+		elseif node.name == "stairs:stair_cobble" then
+			minetest.set_node(pos, {name = "stairs:stair_mossycobble", param2 = node.param2})
+		end
 	end
 })