Check UI string variables match.

[microbit-matt-hillsdon]

Would have caught https://github.com/microbit-foundation/python-editor-next/issues/956 No additional errors. I've also manually reviewed the ICU plural strings, and while a surprising number don't seem to be translated there are no structural errors.

[github-actions[bot]]

Preview build will be at https://review-python-editor-next.microbit.org/check-ui-variables/