[Ram-Pavith]

Adds a JPEG XL converter example (examples/convert_to_jxl.rs).

The image crate has no JXL encoder, so the example decodes/validates input with the crate, then encodes via the first available libjxl backend (cjxl, ffmpeg/libjxl, ImageMagick) and checks the output JPEG XL signature.

Verified: cargo build --example convert_to_jxl, and a --lossless run on a sample PNG whose output decodes pixel-exact.

[mstoeckl]

I don't think this is particularly useful as a program by itself (if you want general format conversion that mostly uses Rust, then something like https://github.com/Shnatsel/wondermagick would be more appropriate), nor does this demonstrate any interesting part of how to use the image crate.