Issue #3259355 by mallezie, mondrake, catch, longwave, mglaman, alexpott, xjm, daffie, Mixologic: Always do a full phpstan analysis on DrupalCI

Issue #3178534 by mondrake, klausi, alexpott, longwave, mallezie, Taran2L, mglaman, Spokje, kim.pepper, daffie, andypost, neclimdul, xjm, Gábor Hojtsy, cmlara: Start running PHPStan on Drupal core (level 0)