Using directive for Scala Js options

[romanowski]

Scala CLI allows configuring following options for Scala JS:

• jsVersion
• jsModuleKind
• jsDom

We are missing using directives parser for them similar to one for JavaOptions.

[sjrd]

Also worth pointing out that even the "options for Scala.js" that scala-cli seems to handle are missing several options of Scala.js itself:
https://github.com/scala-js/scala-js/blob/master/linker-interface/shared/src/main/scala/org/scalajs/linker/interface/StandardConfig.scala
including important ones like esFeatures and moduleSplitStyle.

[romanowski]

Per #531 we should also include jsHeader option.

[lwronski]

Closed #332 via #539