SBT support

从根目录的 README、47-49 行附近的 settings.gradle 以及链接的 sbt-spotless 项目开始。查看 Gradle 关于 Scala plugin 的指南和现有的 build 引入模式；完成的标准是将 SBT plugin 作为 plugin-sbt 引入，通过 Gradle 构建以便发布，并从 README 中链接到它。

@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