Add tests for parsing the var() function

[nex3]

See sass/sass#3245

[skip dart-sass]

[Goodwine]

If I had

$args: --c, d;
a {b: var($args..., )}

or

$args: --c;
a {b: var($args...,)}

Should those cases pass with a { b: var(--c, d) } and a { b: var(--c, ) } respectively?
or would both be considered errors?

[nex3]

That's actually tested on line 275. They're both valid, but they ignore the comma, because Sass generally allows trailing commas in argument lists, maps, and so on.

[Goodwine]

I see, I did notice that test but assumed that @function var would make the special handling to work differently, but if both are meant to be similar then this is fine

[Goodwine]

I see, I did notice that test but assumed that @function var would make the special handling to work differently, but if both are meant to be similar then this is fine