Allow using accessible as a function

[neko-kai]

Expose generation of accessible forwarder as a blackbox macro method (blackbox is sufficient, the type is known ahead of time):

Example:

trait LaunchSite[-R] {
  def launchSite: LaunchSite.Service[R]
}

object LaunchSite {
  abstract class Service[-R] {
    def launchNukes: ZIO[R, Nothing, Nothing]
  }

  val launchSite: LaunchSite.Service[LaunchSite[Any]] = accessible
  // def accessible[S]: S = macro ???
}

Combining this with delegate allows service forwarders to be generated in an Intellij-friendly way:

class Proxy(@delegate s: LaunchSite.Service[Any]) extends LaunchSite.Service[Any]

object launchSite extends Proxy(accessible)

/cc @ioleo @mschuwalow