[MID PRI] Universal editor plugins

[jordwalke]

We can take advantage of the esy conventions, by building editor plugins that understand how to source the project environment when you edit files inside of an esy project. That means people would no longer have to install global tools in order for their editors to work correctly. Each project can have their own version of the IDE tools as dev dependencies. This convention could be used for everything from JS/Flow development, as well as ReasonML and C++.