Support Symfony Mailer

I found a lot of code depending on the system.mail configuration which is not used by Symfony Mailer, especially in the WebformEmailProvider class. This needs to be adapted by a check whether Symfony Mailer is active (and then an alternative implementation utilizing Symfony Mailer) or if it should use the old system.

There might be changes in the Symfony Mailer module needed as well, I've linked the one which I found.

@see #3281115: Support sending attachments using symfony_mailer →