Added support for JXL images.
JPEG-XL is the most advanced of the next-generation of image codecs, supporting both lossless and lossy files — with better compression and quality than PNG and JPG respectively.  It has reduced the disk sizes and bandwidth required for many of the datasets I use.

Pillow does not yet support JXL, but there's a plugin as a separate Python library that does (`pip install jxlpy`), and I've tested that this change works as expected when the plugin is imported.

Dataset used for testing, you must `git pull` as loading it from Python won't work until `datasets-server` is also changed to support JXL files:
https://huggingface.co/datasets/texturedesign/td01_natural-ground-textures

The case where the plugin is not imported first raises an error:
```
PIL.UnidentifiedImageError: cannot identify image file 'td01/train/set01/01_145523.jxl'
```

In order to enable support for JXL even before pillow supports this, should this exception be handled with a better error message?  I'd expect/hope JXL support to follow in one of the pillow quarterly releases in the next 6-9 months.