Parse .ensime to launch Ensime server

This is a follow-up of #3 and should fix #13.

In the end I didn't use ensime-lsp-client, because

It's too heavy: I tried adding the dependency and the generated JS was ~400KB bigger, i.e. +50% ❗️ to the current size. It's a lot for an Atom package and will noticeably slow down activation 🐌
It actually doesn't do much: parses .ensime, takes a couple of values from it and formats java arguments to launch the server. This PR achieves the same result in a more economical way.

Most likely it's heavy because of the fastparse dependency, which is used to parse S-expressions. I don't see much point in writing a custom parser for it if you can reuse an existing node package. (I understand the general idea of having a self-contained Scala.js reusable part, but in this particular situation it's not a good solution)

Also there are a lot of little warts in the code of ensime-lsp-client that I would simplify/refactor/remove, but I don't want to rewrite it, if I'm not going to use it anyway (again, because of fastparse).

So in the end I used fwg/s-expression for parsing .ensime. With the added facade and implementation it's +60KB to the resulting JS 🙆‍♂️

laughedelic / atom-ide-scala

Parse .ensime to launch Ensime server #64