Fix string extraction for Webpack function renaming

[aduth]

🛠 Summary of changes

Updates Webpack i18n string extraction to match on a pattern of Webpack renamed source code for references to t in @18f/identity-i18n.

Example source:

 children: i18n_t('in_person_proofing.headings.cta')

This isn't an especially resilient solution, but since we don't currently operate on the raw source of the files, we have to try to extract with some transformations from Webpack and Babel applied (see related #6359).

Related Slack thread: https://gsa-tts.slack.com/archives/C03FA4VBN76/p1737138493859829

📜 Testing Plan

Verify that production-optimized build includes previously-missing strings in output.

NODE_ENV=production yarn build
find ./public/packs -type f -name 'document-capture-*.en.js' -exec grep "in_person_proofing.headings.cta" {} +

The result of the find command should be non-empty. (On main, the result is empty)

[solipet]

Tested the regex change witih https://regex101.com/r/DBJqyc/1

[shanechesnutt-ft]

Ran the production build locally and everything looks good to me!

[eileen-nava]

Thanks for fixing this. 🙏🏻