fix: hsl and rgb with alpha channel parsed incorrectly

[bpajor]

Description

The original reason of this change is an error with message: Uninitialized string offset 3.

The syntax that was causing the error was:

background-color: hsl(0 0% 10% / calc(1 - var(--hover, 0) * 0.25));

that is being used in many clients css.

The problem was that we were not correctly indexing the content of the function, so that calc was unreachable and was causing the error.

Hovewer fixing the issue has showed in result, that we were not unable to correctly parse the hsl or rgb functions with alpha channel (after /). They were translated to the same functions but with 4 arguments which is incorrect.

With that PR we can fix that problem by discovering when we have hsl or rgb with alpha channel and convert it to hsla or rgba functions, which can take 4 arguments (with alpha channel).

Usage of / is explained here:

• https://developer.mozilla.org/en-US/docs/Web/CSS/color_value/hsl
• https://developer.mozilla.org/en-US/docs/Web/CSS/color_value/rgb

Testing

Unit tests are extended to cover the change, checking the hsl and rgb with an alpha channel.