Implement idx for value sets

[hgiesel]

The last contiguous number on a value set is its idx:

[[$c1|abc|def]] Has a name "c", a sub "0",  and an idx "1".
[[$c2|foo|bar]] Has a name "c", a sub "0", and an idx "2".
[[$meh|foo|bar]] Has a name "meh", a sub "0", and an idx "null".

$eval($c1) => evaluates c1
$eval($c*) => evaluates c1, and c2
$eval($c*:*:0) => evaluates c1, and c2 to their first value.