Add DataDog Scala.js crossbuild

[LukaJCB]

In the context of https://github.com/typelevel/feral it would be fantastic to add support to some of the backends for Scala.js. We have an acute need for DataDog and are working on a Scala.js version right now and would love to upstream it. Thoughts or concerns?

[armanbilge]

Cool! Are you writing a pure Scala client or wrapping their JS client? See also #695 (comment).

[AlexITC]

@LukaJCB I wonder if you ended up publishing the Scala.js module.