Generate static forwarders for object members in companion interface

[smarter]

Same change implemented five years ago in Scala 2:
scala/scala#5131

Fixes #13572.

[smarter]

Ah yeah, this adds public methods to our API even if they're only visible from Java, so it'll either have to go into a new RC for 3.1.0 (it's a regression from scala 2 but not from scala 3, so not sure if we should backport it) or wait until 3.2 /cc @Kordyjan

[jlprat]

My 5cts. I would appreciate if this could go into a new RC for 3.1.0. My reasoning is that Java code using Scala libraries would break when they start using the Scala 3.0 compiled version. This way, the window where the Java compatibility breakage for this case would be minimized to only 3.0.x.

[mdedetrich]

I also agree with @jlprat , "technically" this may not be a regression of the Scala 3 spec however the whole of Scala is that its meant to work with Java so I would agree with getting this out in 3.1 because otherwise it means that any Scala library compiled with 3.1 potentially cannot work with Java.

There have also been cases (albeit incredibly rare) where a Scala2 release was made that created issues with produced binaries and a re-release was made (and it was basically told to the community to ignore the release).