task: #3625890 Commit the yarn configuration and lock file

Issue: https://www.drupal.org/i/3625890

Changes

CI runs corepack enable && yarn install, but none of the configuration that needs was committed: no packageManager to pin a version, no .yarnrc.yml (CI wrote one at runtime), and no lock file, so every pipeline re-resolved the whole dependency tree.

• Pins "packageManager": "yarn@4.9.3", matching ace_editor, admin_audit_trail and canvas_override.
• Commits .yarnrc.yml with the node-modules linker the CI comment says yarn 4 requires.
• Commits yarn.lock.
• Stops ignoring yarn.lock. package-lock.json stays ignored, since this project uses yarn.

Testing

This pipeline is the check that yarn install still resolves and the suite passes with the tree pinned.

Note on ordering: the varbase-e2e 2.0.7 bump on this project is a separate merge request. Both touch package.json on different lines, so whichever merges second may need a rebase. The lock here already resolves 2.0.7, which ^2.0.4 permits, so the two are consistent either way.

AI-Generated: Yes

Checkpoints:

• File an issue
• Addition/Change/Update/Fix
• Testing to ensure no regression
• Automated unit testing coverage
• Automated functional testing coverage
• UX/UI designer responsibilities
• Readability
• Accessibility
• Performance
• Security
• Developer Documentation
• User Guide Documentation
• Reviewed by human
• Code review by maintainers
• Full testing and approval
• Credit contributors
• Review with the product owner
• Release notes snippet
• Release