Add a suite-wide FunctionalJavascript behavior test and Drupal.org CI pipeline

Problem/Motivation

The Plus Suite modules have per-module Unit and Kernel tests, but nothing exercises the suite together through a real browser the way a user builds a page — entering Edit Mode, placing a block, editing a field inline. Because the modules are co-developed and integrate tightly (navigation_plus, lb_plus, edit_plus, tempstore_plus, twig_events, field_sample_value), an integration break between them currently goes uncaught until manual testing.