Add Kotlin integration

[0xnm]

This PR solves Part 1 from https://github.com/com-lihaoyi/mill/issues/3451:

• It in-sources https://github.com/lefou/mill-kotlin with minimal changes
• It adds basic samples

I still need to finalize it (especially automated test for example 4-builtin-commands fails right now), but most of the work is done.

Things I noticed in the original implementation (https://github.com/lefou/mill-kotlin):

• Kotlin stdlib is added by overriding ivyDeps, meaning that if the user of KotlinModule wants to add its own deps and forget to call super.ivyDeps() ++ <their deps> and just does def ivyDeps = <their deps>, then no stdlib will be in the classpath during the compilation and it will fail with cryptic error. Such approach is thus error-prone.
• No Kotlin Reflection library is added automatically to the compile classpath, meaning that if code to compile contains any Kotlin Reflection classes, compilation will fail (because -nostdlib flag is used for compilation)

@lihaoyi Feel free to give your feedback while this is in a Draft state if you want.

[lihaoyi]

I think not supporting kotlin-reflect is fine for this initial PR for Part 1 of the bounty. Part 2 of the bounty covers a lot more scenarios, so we can look at figuring out supporting kotlin-reflect and other fanciness (compiler plugins?) then, as the counterpart to example/javalib/module/6-annotation-processors and example/scalalib/module/6-scala-compiler-plugins

[0xnm]

@lihaoyi Code-wise this PR is done, I think. I addressed your suggestions. The only remaining thing is to add Kotlin samples to the generated docs.

[lihaoyi]

The code looks good I think. The example tests just need to be tweaked I think to match the stdout format of the kotest library. You can run them locally via ./mill -i 'example.kotlinlib.basic[1-simple].local' and adjust them accordingly, and once they're green I'll merge this and wire the example tests into the docsite myself

[lefou]

• It in-sources https://github.com/lefou/mill-kotlin with minimal changes

Don't forget to address copyright requirements. mill-kotlin is not MIT licensed.

[0xnm]

@lefou Yes, sure, it was on my to-do list before marking PR as final, after code changes are complete.

I added the copyright now in https://github.com/com-lihaoyi/mill/pull/3514/commits/85c1f30650682a68e2bd609eca49566e93b7b020. However, since https://github.com/lefou/mill-kotlin has no explicit copyright statement neither in the source files, nor in the license file, I had to come up with the following (since I traced back the first commit to 2020 and there are 2 contributors):

/*
 * Copyright 2020-Present Original lefou/mill-kotlin repository contributors.
 */

Feel free to suggest another one if needed.

Regarding the refactoring: I merged KotlinModulePlatform in KotlinModule in https://github.com/com-lihaoyi/mill/pull/3514/commits/3385f948245ebd30e7c0f16e63e6f1ee41e5e2d1, but I would refrain from the further changes, because I think it is better to keep the focus of this PR on the integration part and keep the original files as-is for that. Refactoring can be done in the further PRs if needed.

[0xnm]

@lihaoyi I fixed Kotlin tests, but there is one job ('integration.{feature,failure}[_].fork.testCached) which failed and I guess it is not related to this PR. Probably there is a flaky test there, but I have no rights to re-run the job (without pushing empty commit).

[lihaoyi]

I re-ran the CI job, looks good. I'll go ahead and merge it and add a landing page for it, and will include a reference to the original repo as @lefou requested. @0xnm you can email me your bank transfer details and I'll pay out the first half of the bounty

[lefou]

What I actually meant to request is to respect the original license, which is Apache License 2. Although it allows you to do almost everything, it also demands to point it out and to include the license text somewhere in the project, if you use or derive the work.

The fact that the actual code you included is from me is rather secondary here. My point is about free and open source work in general. It's only free if you comply to the license, whether you like it or not. And since this is missing in this pull request, I had rather not accepted this PR as-is.