Symfony version(s) affected: at least since 4.4.0

Description
When using Xdebug 3 with develop mode disabled (i.e. for coverage in CI), a warning is triggered inside Symfony\Component\ErrorHandler\Error\FatalError due to this like of code:

How to reproduce

• install Xdebug 3 and enable it WITHOUT develop mode
• install the Error Handler and enable it
• throw an exception and do NOT catch it

Warning: Function must be enabled in php.ini by setting 'xdebug.mode' to 'develop'

Example of failure: https://github.com/getsentry/sentry-symfony/pull/475/checks?check_run_id=2247826488#step:9:25

So, function_exists is no longer good enough for safeguarding such usage. Also, it seems that checking for xdebug.mode from the ini is not good enough too: https://xdebug.org/docs/develop#mode

You can also set the mode by setting the XDEBUG_MODE environment variable on the command-line; this will take precedence over the xdebug.mode setting, but will no change the value of the xdebug.mode setting.