chore: gitignore the js3 fixture output

The one-error-one-warning-one-* fixture configs build into
test/fixtures/js3 by design, so every logging run recreates it.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: bjohansebas

.gitignore

@@ -19,3 +19,4 @@ Thumbs.db
 *.sublime-project
 *.sublime-workspace
 yarn.lock
+/test/fixtures/js3