Upgrade to JSDom v16+

[mourner]

Currently GL JS uses jsdom v13.2.0 (Sep 2019) in dev dependencies. To keep deps up to date, we need to upgrade to a newer version, but v16 (Jan 2020) is a big overhaul that introduces a major breaking change: different instances of JSDOM window don't share the same prototypes of classes, meaning that our approach to resetting window stubs by creating a new instance (see window.js) no longer works properly — breaking about 700 unit tests. Upgrading won't be trivial, but it's necessary if we want to keep GL JS dev environment up to date.