Scala CLI temporary fork of snailgun

This temporary fork adds a few features to the original snailgun library. These changes are motivated by how Scala CLI uses snailgun.

This fork allows

not to supply a stdin InputStream (and not spawn a dedicated thread for it down-the-line)
to pass a thread pool, that snailgun can use to watch for output coming from the nailgun server (rather than spawning a new thread for each command sent to the server)

Building

This fork replaces sbt by Mill. Build it with

$ ./mill __.compile

Run tests with

$ ./mill __.test

This fork JARs are pushed to Maven Central. Depend on it from Mill with

def ivyDeps = super.ivyDeps() ++ Seq(
  ivy"io.github.alexarchambault.scala-cli.snailgun::snailgun-core:0.4.1-sc1"
)