A callback type for ChildProcess.exec.
It receives three arguments: error, stdout, stderr.
On success, error will be null. On error, error will be an instance of Error
and error.code will be the exit code of the child process, and error.signal will be set
to the signal that terminated the process (see ChildProcessExecError).
Alias
alias for 
(error:Null<js.node.ChildProcessExecError>, stdout:haxe.extern.EitherType<js.node.Buffer, String>, stderr:haxe.extern.EitherType<js.node.Buffer, String>) ‑> Void