Applies to javascript files inside the platform library folder, the
exposed Engine code, and any javascript files in modules.
Files ending with ".externs.js" will be ignored, you can create a
".eslintignore" file to specify extra files to be ignored.
Removes AppVeyor and all Travis jobs but the iOS one, which hasn't been ported
to GitHub Actions yet (should be done soon).
Backports new style scripts from `master` branch too to do the same checks.
