Add code linter to JavaScript codebase (and integration suite)

I think it would be a good idea to include a code linter as part of the integration suite and tests. It would ensure that everyone writes consistent looking code across different platforms.

cc: @minrk @ellisonbg @Carreau @jdfreder and anyone else that has a thought

Start by reviewing the JavaScript codebase and the existing integration suite and tests to understand how checks are currently run. Choose and configure a linter that works across supported platforms, integrate it with the suite, and confirm that the tests enforce consistent JavaScript style.

Written by the indexing model from the issue text.