Integrate lefou/`millw` and `millw.bat` into Mill repository

[lefou]

• https://github.com/lefou/millw

Goals:

• Keep all features of lefou/millw, except the --mill-version option
• Shell script needs to be POSIX shell compatible
• New Windows Batch script
• Remove parsing of (arbitrary) local mill script as fallback for an undefined Mill version

[JD557]

While I'm not qualified to help here, if the plan is to have a new windows .bat file, wouldn't it be better to just use a .ps1 file instead?

FWIW, Powershell comes installed in all windows versions since windows 7, and using that should avoid some of the legacy craft from batch.

[lefou]

I find it essential, that we support booting all older Mill versions, so that any old project can simply use the latest version of the script.

[lihaoyi]

@lefou I'd like to revisit this issue, in the run up to Mill 0.12/0.13

IIRC one point of contention was the globally installed mill executable parsing a project-specific ./mill script to try and find the version to use. I used to think this was important, but I've changed my mine and would be willing to give that up now. Since we're now recommending usage being of ./mill bootstrap scripts, the exact behavior when the global mill and local ./mill diverges seems less important now than it used to be

Do you recall if there were any other points of contention? Or what other major ways in which millw diverges from the existing mill script?