Limit number of Scala.js linker tasks that can run at once

[DavidGregory084]

We should limit the number of Scala.js linker tasks that can run concurrently once Scala.js 0.6.27 is released:

Global / concurrentRestrictions += Tags.limit(ScalaJSTags.Link, 2)

This is in the 0.6.x branch of the Scala.js plugin now but not released as yet.

I'm not sure this will resolve all of the memory usage issues that we have been experiencing with the cats build but it should help.

[DavidGregory084]

This doesn't seem to cause a problem anymore as of #2737