Parse alpha value from rgba/hsla colors when using variables

[thecrypticace]

When given colors like: rgba(var(--color), 0.1) or hsla(var(--color), 0.1) we were not parsing the alpha value from them properly. This could result in outputting utilities like the following:

.text-primary\/50 {
    color: rgba(var(--color) 0.1 0.5);
}

Which has 1) the wrong syntax; and 2) too many values because the alpha value wasn't replaced.

This PR fixes addresses this issue and outputs this instead:

.text-primary\/50 {
    color: rgba(var(--color), 0.5);
}