react-express-mongodb: lint sources

to fix some syntax issues in the existing code, the commit
put in place prettier and format the code

Signed-off-by: Jérémie Drouet <jeremie.drouet@gmail.com>

react-express-mongodb/backend/package.json

@@ -5,6 +5,8 @@
   "main": "index.js",
   "scripts": {
     "start": "node server.js",
+    "format": "prettier --write {**/,}*.js",
+    "lint": "prettier --check {**/,}*.js",
     "dev": "nodemon server.js"
   },
   "author": "Syed Afzal",
@@ -22,6 +24,7 @@
     "validator": "^10.1.0"
   },
   "devDependencies": {
-    "nodemon": "^2.0.3"
+    "nodemon": "^2.0.3",
+    "prettier": "^2.0.5"
   }
 }