Support ESFeatures in Scala.js

[lolgab]

ScalaJSModule currently supports the useECMAScript2015 settings.
Nowadays Scala.js offers support for multiple ESVersion.
In the official Sbt plugin they are set inside an object called ESFeatures that contains other settings.
This PR tries to mimic the Scala.js API.
It is a breaking change, since overriding the old useECMAScript2015 doesn't have any effect anymore.

How deprecate overriding useECMAScript2015

Since we can't derive ESFeatures from useECMAScript2015, I can't use the same deprecation mechanism as testFrameworks. What I did instead is to define a private boolean var in ScalaJSModule called overriddenUseECMAScript2015 that is first set to true. It is then set to false by the default implementation of useECMAScript2015 which calls back the new esFeatures. Before fastOpt and fullOpt I call a private checkDeprecations task, which calls useECMAScript2015() and then checks the overriddenUseECMAScript2015. If it was overridden then the value remains true and it fails the task with an error message; otherwise it finds false and continues with the build.

[lefou]

@lolgab You may want to look into JavaModule.bspCompileClassesPath, where I've implemented an override detection for the compile target.

mill/scalalib/src/JavaModule.scala

Line 274 in f0358b9

if (compile.ctx.enclosing == s"${classOf[JavaModule].getName}#compile") {

Once we know we have an overridden deprecated target, we can issue a warning message and also use the value to derive the new target, otherwise we can just go with our defaults.

[lefou]

LGTM, thanks!