Port IOApp.Simple to CE2

[kubukoz]

In series/3.x, a simpler interface for IOApp is provided, for the users who don't need args or exit codes:

object Main extends IOApp.Simple:
  def run: IO[Unit] = IO.println("hello")

I think it could be backported to CE2 for the poor souls who won't be able to migrate for a while for reasons.

[djspiewak]

Great idea!

[alexandrustana]

I could try and solve this one

[djspiewak]

Go for it!

[kubukoz]

Closed in #1759, no idea why github didn't figure that one out.