Imagecropauto feature

Does SimpleImage supports [imagecropauto](https://www.php.net/manual/en/function.imagecropauto.php) (as called in GD) to remove white (or black, etc) background?

$cropped = imagecropauto($image, IMG_CROP_WHITE);

Start by reading SimpleImage's existing image-manipulation API and the PHP GD imagecropauto documentation linked in the issue. Determine how the requested white or black background cropping should be exposed, then verify that the resulting behavior matches the GD function for the stated crop modes.

Written by the indexing model from the issue text.