Add bundlewatch to track bundle size changes in PRs (#2695)

(Below steps already performed, but kept for future reference.)

Head here to get auth ID: https://service.bundlewatch.io/setup-github
Create repo secret called `BUNDLEWATCH_GITHUB_TOKEN` with the token inside

js/package.json

@@ -22,6 +22,7 @@
     "webpack-merge": "^4.1.4"
   },
   "devDependencies": {
+    "bundlewatch": "^0.3.2",
     "husky": "^4.2.5",
     "prettier": "2.0.2"
   },