Support Scalafix migrations in Mill projects

[fthomas]

Scalafix migrations are currently not supported in Mill projects. The corresponding MillAlg#runMigration method is a NOP.

Full support for Scalafix migrations is currently only available for sbt projects. The corresponding code in SbtAlg can be used as inspiration for supporting them in Mill.

[bjaglin]

@joan38 is that something you could be interested in contributing to bring mill on par with sbt? it might not be as trivial as #3564, but I guess it shouldn't be too hard

[joan38]

@bjaglin I currently have a lot on my plate but maybe in the coming weeks I can have a look.