Allow channel string in rgba()

[emielbeinema]

This PR aims to add support for channel strings, fixing for example test

<===> no_alpha/var/arg_1/input.scss
a {b: rgba(var(--foo) 2 3)}

<===> no_alpha/var/arg_1/output.css
a {
  b: rgba(var(--foo), 2, 3);
}

<===>

These changes decrease the number of failing tests with 18.

[AxelMontini]

hmmm it only needs to be cargo fmt'd, can be fixed quickly

[kaj]

Seems reasonable and fixes some checks.

Thank you for the contribution!