Better Scastie support

[JD557]

It would be nice to have a cleaner API for demos like this:

https://scastie.scala-lang.org/mt95s4WbSgqip5YezoLgjQ

[JD557]

This is now unblocked

Here's a more recent example (since the old example is gone):

https://scastie.scala-lang.org/JD557/sXkQ2yQsSpqoQgXounejXQ/4

[JD557]

Snake game example: https://scastie.scala-lang.org/JD557/p9BV2G8LTNKxZjPeoS75vA/5