DEV: adds prettier (#5956)

Run `prettier --write "app/assets/stylesheets/**/*.scss" "plugins/**/*.scss"` after making sure you installed it with `yarn`

It's recommended to configure your editor to run prettier on file save.

package.json

@@ -11,6 +11,7 @@
     "chrome-launcher": "^0.10.2",
     "chrome-remote-interface": "^0.25.6",
     "eslint": "^4.19.1",
+    "prettier": "1.13.4",
     "puppeteer": "^1.4.0"
   }
 }