Cross-compile to Scala 3

[Z1kkurat]

Notes:

1. I had to duplicate common code and remove scala directory, leaving scala-2and scala-3 to make coveralls happy as it considered src/main/scala to be a parent of src/main/scala-2, see run https://github.com/evolution-gaming/play-json-tools/actions/runs/4907729806/jobs/8763025284?pr=34
2. Had to remove scalacOptsFailOnWarn := Some(false) as there's no version of the compiler-opts plugin for Scala 3 and I failed to figure out how to work around SBT to pass it only to Scala 2 builds. Inserted @nowarn manually
3. All modules of the project are cross-compiled!

[coveralls]

coverage: 72.222% (-2.3%) from 74.561%
when pulling a29f47a on generic-scala3
into bb1ceb3 on master.

[plokhotnyuk]

@Z1kkurat Will it solve missing com.evolutiongaming:play-json-jsoniter_3:x.y.z since v0.10.3?

[Z1kkurat]

@Z1kkurat Will it solve missing com.evolutiongaming:play-json-jsoniter_3:x.y.z since v0.10.3?

@plokhotnyuk I'm not sure. That specific module is already declared to be cross-compiled to Scala 3, so if the artifact is missing then it means publish command doesn't take it into account (I have next to no idea how it works in relation to these settings) and it had to be published manually. My understanding is that declaring Scala 3 for all modules can solve this.