patch 9.1.1080: filetype: Mill files are not recognized

patch 9.1.1080: filetype: Mill files are not recognized

Problem:  filetype: Mill files are not recognized
Solution: detect '*.mill' files as scala filetype
          (author)

In the past [Mill](https://mill-build.org/mill/index.html) build files
were always `build.sc` and treated as Scala files. However as the 0.12.x
series of mill you can create a `build.mill` file. You can see a lot of
examples of this if you search
[GitHub](https://github.com/search?q=build.mill&type=code). This small
change just ensures that if you have a `*.mill` file it treats it as a
Scala file.

closes: #16585

Signed-off-by: Chris Kipp <ckipp@pm.me>
Signed-off-by: Christian Brabandt <cb@256bit.org>