add min_light and max_light settings and amend crop registered_plants tables

crops/wheat.lua

@@ -234,8 +234,8 @@ minetest.register_node("farming:wheat_8", table.copy(def))
 farming.registered_plants["farming:wheat"] = {
 	crop = "farming:wheat",
 	seed = "farming:seed_wheat",
-	minlight = 13,
-	maxlight = 15,
+	minlight = farming.min_light,
+	maxlight = farming.max_light,
 	steps = 8
 }