Add option to use Mill Build tool

[softinio]

Mill Build tool seems to be getting traction and used by more people.

Would it be an idea to add option to use Mill or SBT? Letting users choose which they want.

[valencik]

A big part of typelevel.g8 is sbt-typelevel, so we'd need to figure out what that looks like in a mill build setup, which doesn't seem trivial.

I'm not against this, but I am not sure it is worth the effort.
Additionally, to my knowledge no Typelevel org projects currently use Mill.

Just because I found it while searching around, here is the mill g8 template:

• https://github.com/com-lihaoyi/mill-scala-hello.g8