Add a function that infers and fills __all__ from module globals

After some discussion on Discord, an interesting idea came up that could make a good addition to this library. Some people don't like @public decorators and would rather have a single function they can call at the bottom of their module which would look at the module globals, infer the public names (e.g. everything that doesn't start with an _) and fill __all__ automatically.

This function could be something like infer_public_names() or public_all_the_things(). If you're interested in this, let's bikeshed the name. The implementation should be pretty easy.

To start with, I'd give very little configurability for the inferring criteria, and just use non-underscored names. If there's demand we can always add features like regex matches, or other ways to infer, but I think the default will cover 99% of cases, and of course there's still the decorator.