refactor: LineItemService migration to TS + refactoring + fix (#1254)

* refactor(medusa): LineItemService migration to TS + refactoring

* feat(medusa): Cleanup line-item service

* feat(medusa): Rebase develop

* test(medusa): Fix integration cart tests

* fix(medusa): Cart service updateUnitPrices

integration-tests/api/__tests__/store/cart.js

@@ -1739,7 +1739,7 @@ describe("/store/carts", () => {
         .catch((err) => console.log(err))
 
       // Ensure that the discount is only applied to the standard item
-      const itemId = cartWithGiftcard.data.cart.items[0].id
+      const itemId = cartWithGiftcard.data.cart.items.find(item => !item.is_giftcard).id
       expect(cartWithGiftcard.data.cart.items).toEqual(
         expect.arrayContaining([
           expect.objectContaining({