Add support for exporting to `build.mill`/`build.mill.scala` with `export --mill`

[Gedochao]

Is your feature request related to a problem? Please describe.
When exporting to Mill 0.12+, I'd like to be able to get the new build configuration files with .mill and the up-to-date syntax.

Describe the solution you'd like
I wonder if we should just default to build.mill.scala for 0.12+, or maybe it should be behind a --mill-extension flag with .mill.scala set as default... leaning towards the former.

Describe alternatives you've considered
Technically we can keep it the way it is for now, but we'll have to do this to support Mill 1.0.0 very soon, anyway.

Additional context

• Bump mill to 0.12 #3691
• https://github.com/com-lihaoyi/mill/blob/main/changelog.adoc