Platform-cross-aware GHA steps

[armanbilge]

When you define an sbt step in sbt-gh-actions, it automatically prefixes the step with e.g. ++2.13.7 to set the Scala version as defined in the matrix.

When a matrix is split on platform (JVM/JS/Native), it would be helpful to have a step prefixed with e.g. project rootJVM or project rootJS with whatever is defined in the matrix.

I think this is possible, something like this: just as we have axes for OSes, JVMs, and Scala versions in the matrix, we need an axis of "aggregate projects". Each aggregate project would select a slice of sub-projects in the build (e.g., JVM ones or JS ones) and scope all the sbt steps in the workflow to that project, just like they are scoped to a particular Scala version.

I think this might also fix sbt-gh-actions' interop with sbt-project-matrix, which creates one sub-project per Scala version and thus wreaks havoc.

Linking to #18.