Support Scala.js linker config OutputPatterns

[shishkin]

In order to be able to run ESModules on Node without tinkering with package.json, linker output needs to have .mjs extension instead of .js. Scala.js linker supports this via OutputPatterns. Mill ScalaJSModule doesn't expose that config in its API.

Also, would it make sense to expose Scala.js linker API directly instead of the current facade API?

[lolgab]

Also, would it make sense to expose Scala.js linker API directly instead of the current facade API?

This is not possible because we need to support multiple versions of Scala.js and by depending on it directly you lose this property.