Change useMapsLibrary to return the library objects instead of a boolean

[usefulthink]

In order to be closer to the implementation in the google maps API, we want to change the returned type for the useMapsLibrary hook from boolean to the actual API Object returned by google.maps.importLibrary. At the same time we want to drop support for importing multiple libraries at once.

This removes the requirement to access the maps API via the global google.maps. variables (which seems to be something google wants to move away from in the long term).

So instead of

const libraryLoaded = useMapsLibrary('places');

useEffect(() =>{
  if (!libraryLoaded) return;

  const svc = new google.maps.places.PlacesService();
  // ...
}, [libraryLoaded]);

it would be

const placesLibrary = useMapsLibrary('places');

useEffect(() =>{
  if (!placesLibrary) return;

  const svc = new placesLibrary.PlacesService();
  // ...
}, [placesLibrary]);