chore(core-flows): reorganize folder structure for promotion workflows (#6243)

workflows folder structure:

```
- src/
  - promotion/
    - workflows/
      - create-promotion.ts
    - steps/
      - prepare-create-promotion-data.ts
```

RESOLVES CORE-1688

packages/core-flows/src/index.ts

@@ -1,3 +1,4 @@
 export * from "./definition"
 export * from "./definitions"
 export * as Handlers from "./handlers"
+export * from "./promotion"