Support for mill build tool

[jk-1]

Please add support for mill build tool. According the search -functionality, there are no hits for mill in the Scala.js documentation. Scala.js documentation should contain instructions how to use mill to build Scala.js applications in a same way as it now instructs how to use sbt.

[sjrd]

PR welcome.. I don't use Mill myself, so I wouldn't know what to write.