Export ApiLibraryKeys for use in client code

[man-trackunit]

This way clients won't have to make their own copy ApiLibrary keys. Not sure if there's a better name than ApiLibraryKeys.

Alternatively a client could extract the type with something like:

type FirstArg<F extends (...args: never) => unknown> = Parameters<F>[0];
type ApiLibrary = FirstArg<typeof useMapsLibrary>;

But I'd consider that a hack. Better to just export the type imho. 🙏

[usefulthink]

This is actually a fix for a shortcoming in the @types/google.maps package. I already raised that issue in their tracker (googlemaps/js-types#95) but haven't heard back so far. It could be that this will get solved for @types/google.maps and you can import it from there. I would much prefer it that way.

If we implement this in this library, I would prefer to export the entire map as well, but also would like to find a better name for it first (maybe GoogleMapsLibraryMap / GoogleMapsLibraryName? what do you think?)

[man-trackunit]

Hmm. Don't have a good suggestion for naming but your reasoning makes sense. I'll close this PR and hope you get through to @types/google.maps thanks!