Support for java12 --enable-preview

I put this in my build.sbt

javacOptions ++= Seq("--release", "12", "--enable-preview")

But the project structure stay on 12  - No new language features

I would expect it to be 12 (Preview) - Switch expressions

Note that SBT doesn't support these options quite well neither (see https://github.com/sbt/sbt/issues/5021)