Simplify the setup for cross-platform cross-version Scala modules

[lihaoyi]

This example contains a minimal example for cross-publishing some modules across {Scala-JVM,Scala.js}X{2.13.10,3.2.2}

https://github.com/com-lihaoyi/mill/blob/main/example/web/6-cross-platform-publishing/build.sc

It's pretty complicated, given that this is a very common workflow: most Scala modules will support multiple Scala versions, and many will support multiple Scala platforms. There are other ways it can be laid out (e.g. a separate Cross for each module we publish) but none of them are any simpler

We should be able to simplify this somehow

[lolgab]

Previous work in this area: https://github.com/lolgab/mill-crossplatform