WCAG 3 / APCA contrast checking

[tobimori]

WCAG 3 will use a new color contrast method called APCA (Advanced Perceptual Contrast Algorithm). This is way more accurate as it uses the perceptually correct contrast.

https://github.com/Myndex/SAPC-APCA/blob/master/documentation/WhyAPCA.md#why-the-new-contrast-method-apca

It would be cool if this is toggable via a Field property.