Add conditional exports data to the package

[Avaq]

This allows Node 14 users to import the modular code straight from "fluture", as opposed to having to deep import from "fluture/index.js".

[Avaq]

This might need to be considered a breaking change. Prior to it, if a Node 14 user would do:

import Future from 'fluture'

Future.resolve (42)

That would work. Because 'fluture' would resolve to the CommonJS module, which contains all functions.

Now, this will throw TypeError: Future.resolve is not a function, because instead of loading the commonjs export, we now load the default modular export from index.js, which is just the Future constructor without any additional properties attached.