BSP Support for Kotlin modules

[lihaoyi]

Splitting this out of #3397

We now support kotlinlib built in to Mill, so we should support Kotlin modules via IntelliJ BSP as well. AFAIK the Kotlin community is mostly on IntelliJ, so we don't need to worry about VSCode for now. And since kotlinlib is now bundled with Mill, we don't need to worry about making the BSP server extendable (#2598), and can just hard-code support for KotlinModule for now

I'm not sure what exactly will need to be done to make Mill/BSP/IntelliJ/Kotlin work. It seems we have MillJavaBuildServer.scala and MillScalaBuildServer.scala, so likely we will need a corresponding MillKotlinBuildServer.scala file. We may also need changes on the IntelliJ side to allow the IntelliJ BSP client to generate Kotlin modules, if it doesn't already support that

[lihaoyi]

@alexarchambault could you look into this when you're done with the current tickets you're working on?

[alexarchambault]

Sure, I was actually in the midst of adding basic BSP integration tests (that would have caught BSP issues in the current state of #3599, and are needed for future BSP developments, I think)