Ensures sbt is only started with Scala versions that are allowed by the build

Simply add the plugin to your project/plugins.sbt:

addSbtPlugin("net.bzzt" % "sbt-strict-scala-versions" % "0.0.1")

When you have specified scalaVersion := "2.12.8" in your build, after loading this plugin you can no longer start sbt with sbt ++2.12.7 shell.

When you have specified crossScalaVersions := Seq("2.12.8", "2.13.0") in your build, you can start sbt with sbt ++2.12.8 shell or sbt ++2.13.0 shell, but not sbt ++2.12.9 shell.