SBT support

@moznion has made a spotless plugin for the sbt buildsystem. I bumped into it the other day, and I began a conversation with the author about the option of upstreaming it https://github.com/moznion/sbt-spotless/issues/22

We should definitely let our users know that it exists by linking to it in the root README. I think it would also be good to upstream it into the spotless repo. The advantage of that is that when we add features / fix bugs in various steps, it would be easier for sbt users to know which versions to use. For spotless users that don't use `sbt`, it helps to have more eyes on the various steps - you never know who might fix a bug or bring a feature.

The downside of having more build steps is that our build gets bigger and slower. Right now, we have the ability to exclude the maven plugin, we could always change it to where all the plugins are excluded, and you have to manually set `SPOTLESS_INCLUDE_GRADLE` or `SPOTLESS_INCLUDE_SBT`, so that developers working on the build system they care about don't get bogged down by the larger scope.

https://github.com/diffplug/spotless/blob/74d964d22636e58e897ff298c6df996e3fac2b69/settings.gradle#L47-L49

The main challenge to upstreaming @moznion's work is just

- a PR with a folder `plugin-sbt`
- add the folder to `settings.gradle`
- get it building via gradle, so that our publishing works https://docs.gradle.org/current/userguide/scala_plugin.html

ルートREADME、47〜49行付近のsettings.gradle、およびリンクされているsbt-spotlessプロジェクトから始めてください。GradleのScala pluginに関するガイダンスと、既存のbuildへの組み込みパターンを確認してください。完了条件は、SBT pluginがplugin-sbtとして組み込まれ、公開用にGradleを通じてビルドされ、READMEからリンクされていることです。

索引モデルが issue の本文から書いたものです。