Switch to Mill

[alexarchambault]

This switches the build to Mill, but also publishes non-shaded JARs as io.get-coursier::interface-no-shading:X.Y.Z (that explicitly depend on coursier and scala-library without shading / proguarding them). These JARs can be useful in native-image contexts, where the shaded coursier and scala-library take space and resources.

[tgodzik]

@alexarchambault I think this changed caused the interface jars to depend on Scala:

> cs resolve -t io.get-coursier:interface:1.0.29-M3                                                                                                                                                                                                                                                                              
  Result:
└─ io.get-coursier:interface:1.0.29-M3
   ├─ org.scala-lang:scala-library:2.13.17
   └─ org.slf4j:slf4j-api:1.7.36
> cs resolve -t io.get-coursier:interface:1.0.29-M2                                                                                                                                                                                                                                                                      
  Result:
└─ io.get-coursier:interface:1.0.29-M2
   └─ org.slf4j:slf4j-api:1.7.36