Implement SIP-61 @unroll annotation

[bishabosha]

see https://docs.scala-lang.org/sips/unroll-default-arguments.html

PR in progress: #21693

[987Nabil]

If unroll leaves experimental in a future release, is there a possibility to back port it to Scala 2? As a lib maintainer (zio ecosystem) I see us dropping Scala 2.12 as soon as sbt 2.0 is there.
But I don't see us or other big libs drop 2.13 any time soon.
And it would be really nice if at least for 2.13 unroll would work for Scala 2/3 cross build libs

[tgodzik]

I don't think we'll be able to port it to Scala 2, but there is basis of the features in the form of https://github.com/com-lihaoyi/unroll which is already used in some libraries.