Support TOP (OFFSET) in GROUP / JSON operators

[AlexKirkouski]

Example for GROUP (in MyCompany) :

smth = DATA INTEGER (Item);
first = DATA INTEGER (Category);
firstSmth(Category ct) = GROUP SUM smth(Item i) IF level(category(i), ct) ORDER DESC smth(i) TOP first(ct);

EXTEND FORM items
    PROPERTIES (c) first, firstSmth
    PROPERTIES (i) smth
;