Currently we handle only first member of multireturn statement.
Fix it processing each element separately.

n.b.: While at this file add vim settings.

 | tarantool> \set output lua
 | true;
 | tarantool> 1,2,3,4
 | 1, 2, 3, 4;

Fixes #4604

Reported-by: Alexander Turenko <alexander.turenko@tarantool.org>
Signed-off-by: Cyrill Gorcunov <gorcunov@gmail.com>
Reviewed-by: Alexander Turenko <alexander.turenko@tarantool.org>
(cherry picked from commit d7cbd007)