Unify code printers for Scala 2 and Scala 3

[keynmol]

As part of #466 I had to implement a code printer that respects indentation, for Scala 3 support.

There's nothing Scala3 specific in there though, and apart from making the generated code slightly larger, it should work the same for Scala 2.

Would be good to unify them, to reduce the number of moving parts.