[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-plugin-permissions/.babelrc

@@ -1,9 +0,0 @@
-{
-  "plugins": ["@babel/plugin-proposal-class-properties"],
-  "presets": ["@babel/preset-env"],
-  "env": {
-    "test": {
-      "plugins": ["@babel/plugin-transform-runtime"]
-    }
-  }
-}