Support for emitting ES6 classes in the JavaScript compiler.
Right now, the JavaScript compiler emits ES3-style classes. This is fine, but I propose that it may be useful to support emitting ES6-style classes for more modern toolchains. If the JavaScript compiler were to support emitting different kinds of modules (as in #267) then it might be natural to also consider adding ES6 classes alongside ES6 modules.

Thoughts? My eventual goal is getting TypeScript output from Kaitai structs, and I see basically two ways of going about it, either treating it as another language or treating it as an extension of JS. The latter actually seems more logical to me, but to get there we would probably need ES6 style classes first.