Make conscript part of sbt itself

This is actually a suggestion. Just crossed my mind that the official documentation already points to conscript to achieve one of the sbt features: [Scripts](http://www.scala-sbt.org/0.13/docs/Scripts.html) (kind of an interesting circular dependency going on here), so I guess it would make sense that the two projects talk to each other about this possibility.

从链接的 sbt Scripts 文档开始，审查 conscript 目前如何与该 workflow 关联。该 issue 未指出文件、测试或实现边界；要完成此 issue，需要在 conscript 和 sbt 项目之间达成一致的集成设计。

由索引模型根据 Issue 内容生成。