Add cell_id_to_vertices/token_to_vertices

[aaliddell]

Return a tuple of the coordinates of the four vertices. See https://github.com/google/s2geometry/blob/bdaaf97c60b3e29c0eb74dbdc66a7a19f1c937f6/src/s2/s2cell.h#L87-L90 and deeper

[jeremyh3]

Thanks for this library - nice job! This enhancement would be a cool add since its one of the primary things you would do to use the library in some mapping applications (show the cell where the point lives). So you would need to get the cell vertices from cell id/level. It possible to do it from points, but its clunkier than just having a function part of the library.