Simplify and expose munit's rootCause util

[valencik]

Munit has a handy utility for unwrapping exceptions to provide better error messages.

rootCause:

  private def rootCause(x: Throwable): Throwable = x match {
    case _: InvocationTargetException | _: ExceptionInInitializerError |
        _: UndeclaredThrowableException | _: ExecutionException
        if x.getCause != null =>
      rootCause(x.getCause)
    case _ => x
  }

This was introduce in #277 where it was ultimately copied from a Scala compiler util.

1. I think we should expose rootCause under private[munit] so that third-party integrations like scalacheck-effect can use it without copying.

2. We should remove the handling of InvocationTargetException and UndeclaredThrowableException, which are not available on ScalaJS.

• Ultimately I don't believe these two exceptions need to be unwrapped to address the issue reported in assert fail throws Boxed Error #275 which only needed unwrapping of ExecutionException
• Similarly, they are not needed to address the same issue in scalacheck-effect Extract real exception for munit with Future typelevel/scalacheck-effect#196
• These shims are now internal to munit meaning downstream libraries would also need to shim them.

I think the Scala compiler util included InvocationTargetException and UndeclaredThrowableException because it was used in signal handling code that dealt with more than just Futures. I think you can see this in the PR that introduced UndeclaredThrowableException to this code: scala/scala@1fcb865#diff-6624212dec6b02c76a7ef6d0a388f0d0f6b9ad71b520020d493d5c4880d9db3fR120