Give in-crate reason about why draft features are being used in a crate

We set the crate specification version to draft versions, if draft features are being used. If we would document which feature is being used, when exporting a crate we could check if, in the meanwhile, this feature is being stabilized and move to stable context versions. We could use an extra entity for machine-readable documentation.

Benefit would be automated conversion to stable versions, if available.

未指定文件、测试或入口点。首先追踪 crate 规范版本的分配位置以及 crates 的导出位置；定义如何在机器可读的元数据中记录 draft feature 的使用，以及稳定化将如何启用稳定版本。

由索引模型根据 Issue 内容生成。