Better support for ScalaJS/ScalaNative matrices

[djspiewak]

If sbt-crossproject is detected, we should generate a project matrix which corresponds to its usage. This is going to be… more than a little tricky. Probably possible though. The point being that people shouldn't have to manually fiddle with the generated build workflow in order to get parallel JavaScript builds.

For the record, these things all work now, it's just that you can get a prettier and more parallel workflow with better matrix support.