[css-ui-4] Added image-set() function to 'cursor' property

[SebastianZ]

This change adds the image-set() function to the cursor property.

It got implemented in Gecko lately and WebKit has support for a prefixed version of it.

Together with the addition of the function I've also adjusted the syntax a little bit for better readability and fixed some links to CSS Backgrounds 3 because otherwise Bikeshed marks them as fatal errors and doesn't compile the spec.

Closes #5831

Sebastian

[tabatkins]

This should just be swapping the <<url>> for <<image>>, right? There's no reason to limit it to solely image-set(), particularly since you can put any <<image>> you want inside of image-set().

[tabatkins]

(See #5834 for the earlier discussion on this.)

[SebastianZ]

Based on Mozilla's implementation issue I mentioned in the other PR and my tests in Firefox Nightly 88, image-set() is actually restricted to URLs for the cursor property, while it also takes gradients in other properties.

So I guess a different version of image-set() with restricted syntax needs to be defined for this case.

But let's see what Emilio has to say in the other PR.

Sebastian

[frivoal]

This has been made redundant by 78fb03b