Add Scala 3 support

[kubukoz]

To unblock Bloop's ability to cross-build sbt-bloop for sbt 2.0 (sbt/sbt#7698).

[kubukoz]

thought: I didn't spend too much time on it, hoping someone with more Mill experience can provide a better understanding and maybe a proper fix. This seems like something that only affects us until we publish the 3.x support.

[kubukoz]

thought: extends AbstractFunction3 was causing a "missing apply method" in Scala 3. Adding that apply was conflicting with the other apply - hence, version-specific sources.

[tgodzik]

Thanks! We might want to switch to JDK 11 since jsoniter dropped support for JDK 8, which should not be an issue for the tools using bloop-config.

Also, looks like scalafix is not passing. We can disable it for Scala 3 if it proves to difficult to fix.

[kubukoz]

Tried just disabling removeUnused for Scala 3 this way:

  def extraArgs = T {
    if(scalaVersion() == scala3) List("---settings.OrganizeImports.removeUnused=false")
    else Nil
  }

  override def fix(args: String*): Command[Unit] = T.command {
    val allArgs = args ++ extraArgs()

    super.fix(allArgs: _*)()
  }

but Mill complains:

[error] bloop-config/build.sc:73:15: Target#apply() call cannot use `value allArgs` defined within the T{...} block
[error]     super.fix(allArgs: _*).apply()
[error]               ^
[error] one error found
1 targets failed

which I don't really understand yet...

[tgodzik]

Should be fixed now, should we do a release now?

[kubukoz]

Can't hurt 😅

[tgodzik]

It should be out!