Migrate to new Scala.js Linker API

[shishkin]

Scala.js linker now supports multiple top-level modules: https://www.scala-js.org/doc/project/module.html.

Currently, Mill fails to link those setup as it is using deprecated Linker API. See #1684.

This PR is my attempt to migrate to the new API that solves the issue.

As I'm new to the project, please let me know what else might be impacted by the change. I've managed to get my original issue with fastOpt resolved. Still need to test the test command as well.

[shishkin]

As expected, I broke the test runner in Scala.js. For some reason TestAdapter is failing to find the framework.

java.lang.RuntimeException: Failed to get framework
    mill.scalajslib.worker.ScalaJSWorkerImpl.$anonfun$getFramework$2(ScalaJSWorkerImpl.scala:171)
    scala.Option.getOrElse(Option.scala:201)
    mill.scalajslib.worker.ScalaJSWorkerImpl.getFramework(ScalaJSWorkerImpl.scala:171)
    mill.scalajslib.ScalaJSWorker.getFramework(ScalaJSWorkerApi.scala:68)
    mill.scalajslib.TestScalaJSModule.$anonfun$testTask$1(ScalaJSModule.scala:259)
    mill.define.Task$TraverseCtx.evaluate(Task.scala:376)
1 targets failed

Will try to debug further.

[shishkin]

Sorry, I'm not able to figure out the tester part without knowledge of scalajs internals. Would somebody be able to provide some direction?

[shishkin]

Fixed in #1714 instead.

[lefou]

Although we don't merged it, thank you anyway! Your contribution is appreciated.

[shishkin]

Thank you 👍

[lolgab]

@shishkin This is now fully solved in Mill 0.10.4