Cherry-pick source compatible Scala 3 changes

[lihaoyi]

Pulls in some changes from #3369 to try and reduce the size of that diff and simplify future backports

[lihaoyi]

CC @bishabosha. For the upstream PR #3369, not sure whether you want to keep rebasing it, or whether just start merging in latest main (which might be less work?). I expect to follow this PR up with a few more PRs cherry picking the source/bin-compatible versions of that PR into main, so may be a hassle to shuffle each of those updates through a long rebase process

[bishabosha]

I squashed a lot of commits in #3369 so that should make it simpler