Add JXL support

This is an experimental extension that converts any JXL (JPEG XL) image file in the page to WEBP.

It uses the following code for conversion:
* https://github.com/niutech/jxl.js

You can find my fork here:
* https://bit.ly/jxl-as-Chrome-extension

Limitations:

* Conversion may take a couple of seconds, especially for large images.
* No support for animated JXL files. Only the first frame is rendered.
* May cause some pages to not load at all. If this happens, please uninstall the extension.

This is a work in progress. More explanation on use cases, tips, and workarounds here:
* https://r360v.blogspot.com/p/jace.html