Saplings need to grow minimum light level 13

It's copied from minetest_game.

mods/default/trees.lua

@@ -16,6 +16,10 @@ local function can_grow(pos)
 	if is_soil == 0 then
 		return false
 	end
+	local light_level = minetest.get_node_light(pos)
+	if not light_level or light_level < 13 then
+		return false
+	end
 	return true
 end