Add _EXTRA vars for PHPStan, Nightwatch and MkDocs
Problem/Motivation
We have {command}_EXTRA variables for CSpell, ESlint, Stylelint, parallel_lint, PHPCS, upgrade_status and PHPUnit. But there are none for PHPStan and Nightwatch. Both of these commands can accept extra arguments such as --debug and --verbose
Does the Pages job also need this functionality? It would be _MKDOCS_EXTRA because mkdocs is the command being run.