Add trimstr/1 function (#3319)

src/builtin.jq

@@ -76,6 +76,7 @@ def fromdate: fromdateiso8601;
 def todate: todateiso8601;
 def ltrimstr($left): if startswith($left) then .[$left | length:] end;
 def rtrimstr($right): if endswith($right) then .[:$right | -length] end;
+def trimstr($val): ltrimstr($val) | rtrimstr($val);
 def match(re; mode): _match_impl(re; mode; false)|.[];
 def match($val): ($val|type) as $vt | if $vt == "string" then match($val; null)
    elif $vt == "array" and ($val | length) > 1 then match($val[0]; $val[1])