Intent to prototype: JPEG XL decoding

[Kagami Rosylight]

JPEG XL (*.jxl) is a new efficient image format being standardized in JPEG committee. The initial support will be only about regular still images and better support will follow. The feature will be behind a flag until the support is considered full and stable enough.

Bug: https://bugzil.la/jpeg-xl

Platform coverage: All

Preference: `image.jxl.enabled`. There will also be a corresponding item inside about:preferences#experimental.