Allow concat in frontend t()-string sources

[p2edwards]

Description

Safely allow string concatenation in frontend translated strings.

Purpose

• Restore some translation strings that were missing
• Improve code formatting flexibility in the future

Details

Supports usage like

   const textContent = __(
     'This translation string is split ' +
     'across several lines to' + // to allow this kind of comment and
     'keep the column width low.'
   );

Without this fix, examples like the above would be missing from the build output without warning.¹ If you compare extracted-strings.json before and afterwards you'll see maybe 8 or 9 translation strings turn up that were previously missing. Some of them can be seen here: GitHub search 🔎

I wrote some more explanation about how this patch works in my upstream PR 🍴 on grassator/webpack-extract-translation-keys.

• The diff there is likely more reviewable than the one here, so read more there if you're curious.
• The version bump here (6.0.0->6.1.0) is because my other patch got merged. I finally got around to upgrading us so I could base the new patch on that.
• Right now we apply this patch with patch-package, but if it gets merged upstream we can change this to just bumping the version number again.

Related issues

Part of #4944

Checklist

1. If you've added code that should be tested, add tests
2. If you've changed APIs, update (or create!) the documentation
3. Ensure the tests pass
4. Make sure that your code lints and that you've followed our coding style
5. Write a title and, if necessary, a description of your work suitable for publishing in our release notes
6. Mention any related issues in this repository (as #ISSUE) and in other repositories (as kobotoolbox/other#ISSUE)
7. Open an issue in the docs if there are UI/UX changes

Footnotes

1. Previously discussed here and here (internal links) ↩

[jnm]

Tested and found several additional strings added to extracted-strings.json