Format .mill files now that Mill uses build.mill extension

[carlosedp]

Mill in it's latest version will use build files with the .mill instead of .sc extension which can be in the root dir or module sub-directories.

Running scalafmt . doesn't format build.mill files automatically unless project.includePaths = ["glob:**.mill"] is added.

It would be good if *.mill files are also formatted.

Ref: https://github.com/com-lihaoyi/mill?tab=readme-ov-file#0120-rc1---2024-09-09