Add script to list messages that may be duplicates

[matthew-white]

I've wanted to search the codebase for duplicate i18n messages for a while, but it felt slightly complicated. I was randomly thinking about it and realized that it wouldn't be that hard. This PR adds a script along those lines, listing i18n messages that look like duplicates.

What has been done to verify that this works as intended?

When I ran the script, it flagged a number of false positives. However, it also successfully identified several duplicate messages: getodk/central#828.

Why is this the best possible solution? Were any other approaches considered?

The script doesn't list duplicates that it considers to be likely false positives. Maybe it'd be nice at some point to add a flag to list all duplicates that the script sees.

Before submitting this PR, please make sure you have:

• run npm run test and npm run lint and confirmed all checks still pass OR confirm CircleCI build passes
• verified that any code or assets from external sources are properly credited in comments or that everything is internally sourced

[matthew-white]

@ktuite, I've tagged you for review, but I think we can wait to review this until after the release goes out.