Update Mill to 0.12.1

[lolgab]

Changes build files to .mill suffix, which requires package definitions (alternatively .mill.scala can be used which is syntax highlighted by more editors and by GitHub).
Since only build.mill and package.mill can contain Modules and Targets, I moved publish.sc to publish/package.mill and settings.sc to settings/package.mill
Since runner is now a member of RunModule (extended by ScalaModule, I reference the runner module using it's package build.runner.
Mill 0.12 doesn't use positional arguments by default. This PR changes calls to task from the command line to pass --argumentName before all parameters.

Dependencies

• Use T.workspace for compatibility with Mill 0.12 alexarchambault/mill-native-image#108
• Pass pwd as a parameter in Upload.copyLauncher alexarchambault/mill-native-image#111

[Gedochao]

@lolgab should we bump directly to 0.12.1?