[FEATURE] Support rgba and rgb (etc.) being aliases

[JakeQZ]

As of CSS Color Module Level 4, rgba is an alias of rgb; likewise, hsla is an alias of hsl.

This change allows any of the above color functions to contain either three or four arguments, with alpha assumed as the fourth, and allowed to be absent.

[JakeQZ]

I don't consider this a bug, because it wasn't a problem until CSS Color Module Level 4 came along.

[coveralls]

coverage: 41.373% (+0.4%) from 40.95%
when pulling 95ce6d1 on feature/css-color-4/function-synonyms
into bc511bb on main.

[JakeQZ]

Another precursor to #755.

[oliverklee]

Also, is this something to backport?

[JakeQZ]

Blocked by #798. I've pushed changes suggested in review, and included the #798 change in those, just to confirm that all CI tests would pass. Once #798 is merged, and this is rebased, that change should drop out of this.

[JakeQZ]

Also, is this something to backport?

If 9.0 is still some way off, then yes, but see below.

This is mutlipart change. The key change to support space-separated arguments with a forward-slash separating the alpha argument is yet to come. It's an enhancement rather than a bugfix, but one that is needed sooner rather than later, since the CSS syntax has evolved.

I noted in a recent backport that some syntaxes had to be changed to support PHP 5.6. OTOH, I recently saw a message from my hosting provider saying that if I wanted to continue using a PHP version below 8.0, I'd have to pay an additional premium.

The current main requires PHP >= 7.2, which itself is well beyond EOL.

I think it would be more than reasonable to cut a 9.0 release fairly soon, regardless of not having completed all that we planned for it, and stop wasting time on the backports (focusing on something more interesting instead).