update build.sbt, fix cross-pub 2.13, add semVer and make build more idomatic

[bjornregnell]

• Sbt idomatic build should include settings inside of lazy val ... (project in file(".")).settings(....

  • Idiomatic build here:
    https://github.com/scala/scala-seed.g8/blob/339fa46feca8e510df394e4775dd69fbb0c28ffd/src/main/g8/build.sbt

• Add SemVer into the build to enable check for binary compatibility etc. More info here:

  • https://www.scala-lang.org/blog/2021/02/16/preventing-version-conflicts-with-versionscheme.html
  • https://www.youtube.com/watch?v=0T3vBnYCXn4
  • https://www.scala-sbt.org/1.x/docs/Publishing.html#Version+scheme
  • https://eed3si9n.com/enforcing-semver-with-sbt-strict-update

• For more info on sbt tricks in the migration guide:
  https://docs.scala-lang.org/scala3/guides/migration/compatibility-classpath.html