feat(*): Add stripe checkout support

src/routes/checkout/+page.svelte

@@ -0,0 +1,14 @@
+<script src="https://js.stripe.com/v3/"></script>
+
+<section>
+	<div class="product">
+		<img src="https://i.imgur.com/EHyR2nP.png" alt="The cover of Stubborn Attachments" />
+		<div class="description">
+			<h3>Stubborn Attachments</h3>
+			<h5>$20.00</h5>
+		</div>
+	</div>
+	<form action="/api/checkout" method="POST">
+		<button type="submit" id="checkout-button">Checkout</button>
+	</form>
+</section>

src/routes/checkout/cancelled/+page.svelte

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+<section>
+	<p>Forgot to add something to your cart? Shop around then come back to pay!</p>
+</section>

src/routes/checkout/success/+page.svelte

@@ -0,0 +1,6 @@
+<section>
+	<p>
+		We appreciate your business! If you have any questions, please email
+		<a href="mailto:orders@example.com">orders@example.com</a>.
+	</p>
+</section>