Add linter for non-functions JavaScript

[samtstern]

Here is the list of applicable files:

$ find . -type f -name "*.js" -not -path "*functions*" | xargs dirname | sort | uniq
./authenticated-json-api/public
./authorized-https-endpoint/public
./email-confirmation/public
./exif-images/public
./fcm-notifications/public
./fulltext-search-firestore/public
./instagram-auth/public
./isomorphic-react-app/src
./isomorphic-react-app/src/containers
./linkedin-auth/public
./nextjs-with-firebase-hosting/src/app
./nextjs-with-firebase-hosting/src/app/components
./nextjs-with-firebase-hosting/src/app/pages
./presence-firestore/public
./quickstarts/email-users/public
./spotify-auth/public
./text-moderation/public
./username-password-auth/public

Context: #333