feat: add CartConditionable interface and first-class exceptions

src/Contracts/CartConditionable.php

@@ -0,0 +1,12 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Wearepixel\Cart\Contracts;
+
+use Wearepixel\Cart\CartCondition;
+
+interface CartConditionable
+{
+    public function toCondition(): CartCondition;
+}