builtin.jq: naive abs/0

manual.yml explains that the def is naive, and mentions fabs, etc.

src/builtin.jq

@@ -10,6 +10,7 @@ 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 del(f): delpaths([path(f)]);
+def abs: if . < 0 then - . else . end;
 def _assign(paths; $value): reduce path(paths) as $p (.; setpath($p; $value));
 def _modify(paths; update):
     reduce path(paths) as $p ([., []];