Support color contrast checking in the computed panel

[xaddict]

Now that many sites are using variables to set colors and background colors it is very hard to pop up the color picker with contrast ratio information in it and easily check whether it is a sufficient ratio. It would be handy if the computed panel supported the contrast ratio checks for `background-color` and `color` and enabled changing the colors (only in the pop-up) to check what change has to be made to get a sufficient contrast.

Also, supports the APCA contrast formula please. Maybe as an option in settings?

[Jon]

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[Honza]

I filed a report in our bug tracker here:

https://bugzilla.mozilla.org/show_bug.cgi?id=1877427

Thank you!

Honza