First class support for Scala.js

[armanbilge]

So far I think there are two primary issues:

1. Scala.js (and native) can use Mima, so actually enable that :)
2. Don't assume that Scala.js only exists in cross-projects. If a project is Scala.js only, it is not detected and all your awesome Scala.js settings don't kick in. E.g. the -P:scalajs:mapSourceURI: stuff which as a result is broken for macrotask executor.