If-Then-Else Syntax support

[tmarsteel]

If-Then-Else Logic is already supported with this "syntax":

?- once((If, Then) ; Else).

The official syntax, used by other prologs, and that must be implemented with this ticket however is

?- (If -> Then ; Else)

The trick is that with iso operators, this is parsed as ';'('->'(If, Then), Else). ->/2 is then probably defined like

'->'(If, Then) :-
  call(If),
  (Then ; !).

With ! being basically impossible to implement into the coroutine-based search tree, implementing this syntax on ISO operators seems pretty challenging.
One option could be redefining the operators so (If -> Then ; Else) gets parsed as '->'(If, ';'(Then, Else)) and implementing '->'/2 using once.

If it is not possible/feasible to implement this way, consider an alternate syntax that is more readable than the once version, e.g. if(IF) then (Then) else (Else)