phpBB / Codespaces support

https://github.com/phpbb/phpbb/pull/6434

By using GitHub Codespaces, we can give all developers (that would like to use it) an identical development environment that is completely pre-configured with a LAMP stack, Xdebug, phpBB installation and VS Code – all hosted in the cloud with no manual setup involved! The IDE and the phpBB installation are hosted entirely on GitHub's servers, and it only needs to run while it's being used. This is a massive time-saver because it can remove the need for a local development environment, particularly useful for new contributors or people who are not making big changes.