Enable moss/lichen/algae spreading (#17)

plants.lua

@@ -163,4 +163,28 @@ minetest.register_abm({
 			grow_caverealms_mushroom(pos)
 		end
 	end,
-})
+})
+
+
+-- spread moss/lichen/algae to nearby cobblestone
+minetest.register_abm({
+	label = "Caverealms stone spread",
+	nodenames = {
+		"caverealms:stone_with_moss",
+		"caverealms:stone_with_lichen",
+		"caverealms:stone_with_algae",
+	},
+	neighbors = {"air"},
+	interval = 16,
+	chance = 50,
+	catch_up = false,
+	action = function(pos, node)
+		local num = minetest.find_nodes_in_area_under_air(
+			{x = pos.x - 1, y = pos.y - 2, z = pos.z - 1},
+			{x = pos.x + 1, y = pos.y + 1, z = pos.z + 1},
+			"default:cobble")
+		if #num > 0 then
+			minetest.set_node(num[math.random(#num)], {name = node.name})
+		end
+	end,
+})