add min_light and max_light settings and amend crop registered_plants tables

crops/cucumber.lua

@@ -59,7 +59,7 @@ minetest.register_node("farming:cucumber_4", table.copy(def))
 farming.registered_plants["farming:cucumber"] = {
 	crop = "farming:cucumber",
 	seed = "farming:cucumber",
-	minlight = 13,
-	maxlight = 15,
+	minlight = farming.min_light,
+	maxlight = farming.max_light,
 	steps = 4
 }