Scala 3

[arashi01]

• Update Scala versions.
• Update sbt, sbt plugins, GitHub Actions.
• Enable Scala 3 build.

[sjrd]

What is the motivation for this PR? As the readme mentions, you probably should not use this library for anything except when talking to scalajs-linker and scalajs-js-envs. Those libraries are built for 2.12 and 2.13 only, by design of how the Scala.js repo is organized. If necessary, they can be used from Scala 3 using cross(CrossVersion.for3Use2_13).

[arashi01]

@sjrd Mainly so we can move on to publishing scalajs-js-envs for Scala 3 if that is the plan, to get rid of the cross(CrossVersion.for3Use2_13) usages.

[sjrd]

The situation is basically the same for scalajs-js-envs: what's the actual issue with for3Use2_13?

I guess if you want scalajs-js-envs on _3, you will also want scalajs-linker. But the latter is very unlikely to get built for _3 in the foreseeable future, due to how scala-js/scala-js is architected; it would create a circular dependency between scala-js/scala-js and lampepfl/dotty, which is not desirable.

From my point of view, it's much easier to keep using for3Use2_13 for this set of core libraries. None of them define any macro nor have they any dependency on any external library, so it's always safe to do so.