Consistently name generated ML binary

[marzipankaiser]

Most backends generate an executable with the same name as the file containing the main. This is not the case for ML, which always generates one named mlton-main:

https://github.com/effekt-lang/effekt/blob/1f0c055f69c0596098e7b135ddc0e408cb08efbb/effekt/jvm/src/main/scala/effekt/Runner.scala#L301-L304

We should change this to make it consistent with the other backends.