[medusa-interfaces] : Adds decorator functionality to BaseService (#39)

Plugins and projects can add decorators in services. E.g. if a plugin needs to load some additional information on carts the plugin can register a decorator via: `cartService.addDecorator(someFunc)` which will be available later through `cartService.runDecorators()`.

packages/medusa-interfaces/package.json

@@ -11,7 +11,8 @@
   "scripts": {
     "build": "babel src --out-dir dist/ --ignore **/__tests__",
     "prepare": "cross-env NODE_ENV=production npm run build",
-    "watch": "babel -w src --out-dir dist/ --ignore **/__tests__"
+    "watch": "babel -w src --out-dir dist/ --ignore **/__tests__",
+    "test": "jest"
   },
   "author": "Sebastian Rindom",
   "license": "AGPL-3.0-or-later",
@@ -21,8 +22,11 @@
     "@babel/plugin-proposal-class-properties": "^7.7.4",
     "@babel/plugin-transform-runtime": "^7.7.6",
     "@babel/preset-env": "^7.7.5",
+    "@babel/runtime": "^7.9.6",
     "cross-env": "^5.2.1",
-    "eslint": "^6.8.0"
+    "eslint": "^6.8.0",
+    "jest": "^25.5.0",
+    "prettier": "^1.19.1"
   },
   "dependencies": {
     "mongoose": "^5.8.0"