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.

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Would this be the new implementation tested? jxl-rs (v0.1.0) or would it be the "JPEG-XL_OXIDE" instead?

And more importantly, will the flag be enabled by default in nightly & dev-builds atleast? That would be very important in allowing for real world tests if more than browser type allowed it by default (aside from Safari or Chromium-like browsers)