Add KtFmt support (500USD Bounty)

[lihaoyi]

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From the maintainer Li Haoyi: I'm putting a 500USD bounty on this issue, payable by bank transfer on a merged PR implementing this.

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This ticket is to add support for Kotlin https://github.com/facebook/ktfmt, in the spirit of our existing support for mill.javalib.PalantirFormatModule and mill.scalalib.scalafmt.ScalaFmtModule. We already have KtLint support which does support autoformatting, but KtFmt seems pretty popular so we should support it too.

Like our support for PalantirFormat or ScalaFmt,

• KtFmt support should come in both a trait that can be mixed into KotlinModules, as well as a standalone external module that can be used on any existing KotlinModules unchanged.
• Should include unit tests and an example test in example/kotlinlib/linting/ that can be included in the documentation
• Should expose any relevant configuration on KtFmt that the user may want to override