Support export-ing to Mill 1.x.y & allow to override Mill version with --mill-version

[Gedochao]

Closes #3882

• extracts pieces of Migrate to Mill 1.0.6 (was 0.12.17) #4019 related to Mill export
• defaults to build.mill.scala for the main build file when exporting to Mill 1.x.y (stays on build.sc for 0.12.x for now, even though later patches support it)
• adds a --mill-version command line option for export, which does what you'd think it'd do
• refactored export integration tests to run them on both Mill 0.12.17 & 1.0.6
• this is just the tip of the iceberg in regard of what Mill export would need to be considered fully stable, but at least it'll now work with modern Mill versions

obvious follow-ups (or a list of things I'd like to do, but probably won't have the time - but then, who knows):

• bring Mill 0.12.17/1.0.6 export up to such a high level of up-to-date-ness, that integration tests don't log deprecation warnings
  • then add a check for those deprecation warnings, so that we don't fall back behind with that
• add support for Mill 1.1.x (it's at 1.1.0-RC3 at the time of me writing this, so it's likely not very far off)
  • run the tests for that as well
• add more test cases
• give a similar amount of love to the other build tools
  • SBT
  • Maven

[tgodzik]

LGTM