[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/.eslintrc

@@ -0,0 +1,9 @@
+{
+  "plugins": ["prettier"],
+  "extends": ["prettier"],
+  "rules": {
+    "prettier/prettier": "error",
+    "semi": "error",
+    "no-unused-expressions": "true"
+  }
+}