Add `find_load` and `find_save`

[jcupitt]

find_load and find_save would be useful for deciding what options are available to new_from_file etc. Take the implementation from eg.:

https://github.com/libvips/pyvips/blob/master/pyvips/vimage.py#L347-L350

See https://github.com/libvips/libvips/issues/2953 for details.