Add chokidar@3 as an explicit dev dependency

Browser tests are failing in the CI environment with:
chokidar@3: Error: Cannot find module 'chokidar'

Don't know why but this thread might be relevant:
facebook/create-react-app#10811

Author: jcmosc

package-lock.json

@@ -69,6 +69,7 @@
     "@types/node": "^14.14.21",
     "chai": "^4.2.0",
     "chai-subset": "^1.6.0",
+    "chokidar": "^3.5.3",
     "cross-env": "^7.0.3",
     "eslint": "^7.18.0",
     "isomorphic-fetch": "^3.0.0",