Allow to specify allowedExports and disallowedExports

[desfero]

Background
Often it's more useful to restrict what can be exposed from the folder/package in the same folder to keep configs more organized. It will also simplify the refactoring of folder structure in complex projects.

Technical notes
With exports rules, we can end up with some performance downsides given we need to parse the whole project structure (up to the root structlint config). Given that let's see if it's the huge problem on for .e.g react codebase.