[implemented] Support AutoCloseable workers (RFC)

[lefou]

The documentation has some example for T.worker:

def webpackWorker = T.worker {
  // Spawn a process using java.lang.Process and return it
}

When used like this, there is potential to hog resources longer then needed. Especially, if the worker depends on other targets and these change.

The idea is, that we check whether the value type T of the Worker[T] is an java.lang.AutoCloseable. In that case, we can call close() before we create a new instance or before we shutdown Mill.

We could also think about supporting scala.util.Using.Releasable type-class.

In the example above, we could implement the close function to stop the previously spawned process.

[lefou]

I implemented this idea in

• Implemented AutoCloseable Workers #1781