Support var() in shorthand and multiple-value functions

[liZe]

Fix #1219.

Possible improvement before merging: support var() in other functions.

Let’s forget for now: support other functions such as calc() (see #357.)

[liZe]

OK, let’s focus on var in functions first, it’s already really complicated.

Who on hell allowed things like

--red: 100; --green: 80; --blue: 0; color: rgb(var(--red) var(--green) var(--blue))

or

--red-green: 100, 0; color: rgb(var(--red-green), 200)

in CSS?

[liZe]

Possible improvement before merging: support var() in other functions.

Done!