fix: add missing prefix for CheckoutSessionItemId

This prefix can be seen in the example data here:
https://stripe.com/docs/api/checkout/sessions/line_items

src/ids.rs

@@ -495,7 +495,7 @@ def_id!(CardId, "card_");
 def_id!(CardTokenId, "tok_");
 def_id!(ChargeId, "ch_" | "py_"); // TODO: Understand (and then document) why "py_" is a valid charge id
 def_id!(CheckoutSessionId, "cs_");
-def_id!(CheckoutSessionItemId: String); // TODO: Figure out what prefix this id has
+def_id!(CheckoutSessionItemId, "li_");
 def_id!(ConnectCollectionTransferId, "connct_");
 def_id!(CouponId: String); // N.B. A coupon id can be user-provided so can be any arbitrary string
 def_id!(CustomerId, "cus_");