Add build.mill as a project file name for the mill project type

[LaurenceWarne]

Hi, this PR adds build.mill as a project file name for the mill project type, it seems to be the recommended extension now, see https://mill-build.org/mill/scalalib/intro.html.

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Thanks!

[bbatsov]

Won't this require some code changes or it was already accounted for?

[LaurenceWarne]

As in allowing lists? - If so I believe it should already be accounted for:

projectile/projectile.el

Line 2934 in db52cf4

(project-files (if (listp project-file)

[bbatsov]

Ah, ok. I just noticed you updated the docstrings, but there were no other changes, so it seemed prudent to ask about this. :-)

[LaurenceWarne]

Ah yeap, probably should have pointed that out, sorry!