Implement add/1 (#3144)

src/builtin.jq

@@ -8,7 +8,8 @@ def unique: group_by(.) | map(.[0]);
 def unique_by(f): group_by(f) | map(.[0]);
 def max_by(f): _max_by_impl(map([f]));
 def min_by(f): _min_by_impl(map([f]));
-def add: reduce .[] as $x (null; . + $x);
+def add(f): reduce f as $x (null; . + $x);
+def add: add(.[]);
 def del(f): delpaths([path(f)]);
 def abs: if . < 0 then - . else . end;
 def _assign(paths; $value): reduce path(paths) as $p (.; setpath($p; $value));