feat: add a line item to an order edit (#2243)

**What**
- Implement adding a line item to order (edit)

**How**
- _by implementing the following "flow"_
  - generate a line item
  - computing line item adjustments for that line item
  - creating tax lines
  - creating a change record

**Testing**
- **_integration tests_**
  - check if line item and order item change objects are created (with correct tax lines)
  - line item adjustments are generated if
    - fixed discount is applied to cart
    - percentage discount is applied
- **_unit tests_** 
  - ensure that methods from Inventory, LineItem, LineItemAdjustment etc. services are called 

---

RESOLVES CORE-495

packages/medusa/src/services/__tests__/line-item.js

@@ -4,7 +4,6 @@ import LineItemService from "../line-item"
 import { PricingServiceMock } from "../__mocks__/pricing"
 import { ProductVariantServiceMock } from "../__mocks__/product-variant"
 import { RegionServiceMock } from "../__mocks__/region"
-
 ;[true, false].forEach((isTaxInclusiveEnabled) => {
   describe(`tax inclusive flag set to: ${isTaxInclusiveEnabled}`, () => {
     describe("LineItemService", () => {