Add scalafmt option to binding generating tasks

[keynmol]

The general plan I have in my head:

1. bindgenRunScalafmt - new Boolean setting, default to false
2. All Scala-generating bindgen tasks check that option and if it's true, then run scalafmtOnly with the list of generated files

We don't need to depend on sbt-scalafmt, we can look up the task by name:

val task  = InputKey[Unit]("scalafmtOnly")

And then refer to it by (Compile / task).toTask(" <list of files, space-separated>").

Can use Def.sequential to do both tasks:

    Def.sequential(
      codeGeneratingTask,
      Def.taskDyn {
        // get list of generated files somehow
        (Compile / task).toTask(s" ${files.mkString(" ")}")
      }
    )

To get list of generated files, we can modify the existing tasks to return a list of files (unless they already do that).