Site Health check that looks for files that reference PHP sessions

WP Launch Check scans for files that reference sessions and recommends [WP Native PHP Sessions](https://github.com/pantheon-systems/wp-native-php-sessions) if anything is found, but this check is not run on the WP Site Health page. The mu-plugin is a good place to pull some of these checks into the WP admin and this would be a good check to have.