Refactor `CompileModule` out of `JavaModule`

[lefou]

It should contain the dependency management targets (ivDeps, compileIvyDeps) and the a generic compile target. It should also support compiler stacking, so there is a clear way how two mixed-in CompilerModules will work together (think: java-before-scala or scala-before-kotlin).

[lefou]

• This is related to https://github.com/com-lihaoyi/mill/issues/2781

[lefou]

I think it would be good to start this idea in a complete different and @experimental package. So we can roll-out and test it as soon as available and also use feedback to settle with a stable API.

[lihaoyi]

Probably going to call this a wontfix for now