feat: add turbopack support to optimize-locales

[stefanprobst]

currently the @react-aria/optimize-locales-plugin does not support turbopack, because turbopack has no full plugin api and no unplugin integration. but turbopack does support loaders, so this pull request adds basically a null-loader for unused locales/messages.

see #8897

✅ Pull Request Checklist:

• Included link to corresponding React Spectrum GitHub Issue.
• Added/updated unit tests and storybook for this change (for new code or code which already has tests).
• Filled out test instructions.
• Updated documentation (if it already exists for this component).
• Looked at the Accessibility Practices for this feature - Aria Practices
• I understand every change in this PR and can explain why it's there.
• If AI-assisted, I followed our AI contribution guidance and pointed my assistant at CLAUDE.md.

📝 Test Instructions:

you can run the github action at https://github.com/stefanprobst/react-aria-optimize-locales-turbopack-repro/actions/workflows/e2e.yml and point it to this branch. it will build two minimal next.js apps, one with this turbopack loader enabled, and one without, and then check whether unused message strings have been excluded.

see this run: https://github.com/stefanprobst/react-aria-optimize-locales-turbopack-repro/actions/runs/31874407939/job/94987870478#step:5:128