Make next/image support JPEG XL

[jfoz]

Goals

1. Make next/image able to use a JPEG XL file as the source image
2. Make next/image able to emit a JPEG XL file as the output image

Non-Goals

No response

Background

JPEG XL is currently supported on Safari, and has just been merged (behind a flag) in Chromium. It typically achieves significantly smaller file sizes than legacy formats like JPEG or PNG.

Having the Image component use it ensures it is loading images as efficiently as possible.

Proposal

• Allow src prop of the Image component to specify a JPEG XL file
• Allow formats to include 'image/jxl'