report diagnostic messages on standard error

PHP diagnostic messages do not interfere any longer with the standard
output of the auto-completion.

whatever the setting of display_errors [1] is, set it to the string
"stderr" (as of PHP 5.2.4) to send errors to the standard error stream [2],
always,  the stream for writing diagnostic output.

this is specifically important as the `symfony-autocomplete` command writes
conventional output to standard output and some _default_ php (incl. cli)
configurations display errors on standard output,  too,  interleaving the
conventional output with error information which renders it flawed.

this did affect the Dockerhub `php` official image (php:8.1-cli-alpine) [3]
and was confirmed with php8.1-cli sury debian package.

usage example (php8.1 -n -f ...):

    $ symfony-autocomplete --shell=bash --aliases=codecept \
        > /etc/bash_completion.d/composer
    Deprecated: Return type of Symfony\Component\Console\Helper\HelperSet::getIterator() should either be compatible with IteratorAggregate::getIterator(): Traversable, or the #[\ReturnTypeWillChange] attribute should be used to temporarily suppress the notice in /home/mot/PhpstormProjects/symfony-console-autocomplete/vendor/symfony/console/Symfony/Component/Console/Helper/HelperSet.php on line 104

PHP diagnostic messages (error, warning, deprecation etc.) are now shown
on standard error - even if not before - according to the error reporting
level [4] of the PHP cli run-time configuration.

php-language-level: 8.1
issue: 72
credits: Alberto Tavoletti (sviluppo; reporting and testing)
[1]: https://www.php.net/manual/en/errorfunc.configuration.php#ini.display-errors
[2]: https://pubs.opengroup.org/onlinepubs/9699919799/functions/V2_chap02.html#tag_15_05
[3]: https://hub.docker.com/_/php
[4]: https://www.php.net/manual/en/errorfunc.configuration.php#ini.error-reporting

bin/symfony-autocomplete

@@ -1,6 +1,10 @@
 #!/usr/bin/env php
 <?php
 
+if (function_exists('ini_set')) {
+    // utility errors on standard error
+    ini_set('display_errors', 'stderr');
+}
 
 if (file_exists($autoload = __DIR__.'/../../../autoload.php')) {
     require_once $autoload;