Bump ESLint's parser options to support ES6 syntax, which allows developers to start using a number of features associated with modern JavaScript.
Doing this now that #2286 has been marked as resolved, since IE 11 support was the last known blocker for such changes.
Adding [eslint-plugin-compat](https://github.com/amilajack/eslint-plugin-compat) to check for usage of incompatible APIs.
For a list of browsers, we start out using both the 'defaults' query along with IE 11, which is still expressly supported in OSD v4 (current).