Add init for sbt

[sake92]

Got basic init setup for sbt working.

Depends on https://github.com/[sake92/sbt-build-extract](https://github.com/sake92/sbt-build-extract) sbt plugin.
Users have to add it to their plugins.sbt file.
The plugin exports each sbt project to root target/build-export folder, in JSON format.

Then mill picks up those JSON files and parses them to build the build.mill and package.mill files.

[lihaoyi]

The approach of using a custom SBT plugin sounds reasonable. Can we include it in this PR and keep it in the Mill repo?

Adding the plugin manually seems like it can be automated by having the command write a file project/mill_export.sbt to include it during the export, and remove the file after it's done

[sake92]

I've embedded it inside this repo:

• added CI to run on PR
• added CD to publish on tag

Also applied project/exportBuildStructure.sbt temporarily as you suggested, works good!

[sake92]

Seems like I didnt copy the sbt plugin sources properly 🤦🏻

But anyways, seems like #4586 got it further.