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

[Pfeil]

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.