Type and signature augmentation of the Java standard library and other libraries

[vandmo]

A bit related to #7871 but I think the question warrants being discussed on a higher level.

Python has https://github.com/python/typeshed and TypeScript has https://github.com/DefinitelyTyped/DefinitelyTyped and both have made a huge impact.

Except for explicit nulls it seems like Caprese would benefit a lot of having information about which capabilities are needed for various functions in the Java API.