Re-enable PHPStan job

Closes #3415291

phpstan.neon

+includes:
+    # The WEB_ROOT environment variable is available on GitLab CI.
+	- %rootDir%/../../../%env.WEB_ROOT/core/phpstan.neon.dist
+
+parameters:
+  ignoreErrors:
+    # Drupal core's PHPStan config file ignores the non-anonymous variant of this.
+    - "#^Anonymous class extends @internal class#"
+  # Drupal core needs to ignore more things than we need to!
+  reportUnmatchedIgnoredErrors: false