Create message JSON files for 'en'

[rtibbles]

Summary

• Due to a change in our webpack build process message files for the 'en' language code no longer get generated at build time
• This updates our message file generation code to create message files for english from the default messages
• Updates our .gitignore to no longer ignore en message files
• Commit the latest en message files

References

Fixes #8908

Reviewer guidance

Do you get any missing translation warnings in English? (most likely culprit is when using a perseus exercise)

---

Testing checklist

• Contributor has fully tested the PR manually
• If there are any front-end changes, before/after screenshots are included
• Critical user journeys are covered by Gherkin stories
• Critical and brittle code paths are covered by unit tests

PR process

• PR has the correct target branch and milestone
• PR has 'needs review' or 'work-in-progress' label
• If PR is ready for review, a reviewer has been added. (Don't use 'Assignees')
• If this is an important user-facing change, PR or related issue has a 'changelog' label
• If this includes an internal dependency change, a link to the diff is provided

Reviewer checklist

• Automated test coverage is satisfactory
• PR is fully functional
• PR has been tested for accessibility regressions
• External dependency files were updated if necessary (yarn and pip)
• Documentation is updated
• Contributor is in AUTHORS.md

[nucleogenesis]

Just spun it up and seeing this - not sure if it's related directly to this PR

[rtibbles]

I think that's this issue? #8829

[radinamatic]

I did about 8 different Perseus exercises and a cursory walk-through other main pages, no missing EN strings! 💯

[radinamatic]

LGTM, !