chore: explicitly add devDependencies for monorepo peerDependencies (#2808)

packages/medusa-plugin-economic/package.json

@@ -22,7 +22,8 @@
     "@babel/runtime": "^7.9.6",
     "client-sessions": "^0.8.0",
     "cross-env": "^5.2.1",
-    "jest": "^25.5.2"
+    "jest": "^25.5.2",
+    "medusa-interfaces": "^1.3.3"
   },
   "scripts": {
     "build": "babel src -d .",