feat(medusa): adds support for gift cards (#132)

* fix(medusa-plugin-brightpearl): adds gift cards to gift card nominal code

* fix: allow orders with 0 total to be created

* fix: gift card ready

* tests passing

* docs: brightpearl comment

* docs: clearer REMEMBER note

packages/medusa/src/services/__tests__/discount.js

@@ -294,6 +294,7 @@ describe("DiscountService", () => {
       expect(DiscountModelMock.create).toHaveBeenCalledWith({
         code: expect.stringMatching(/(([A-Z0-9]){4}(-?)){4}/),
         is_giftcard: true,
+        original_amount: 100,
         discount_rule: {
           type: "fixed",
           allocation: "total",