lint: Add lint rule to warn people making changes to static/locale.

We occasionally get PRs with translations or other fixes to the files in static/locale, due to a reasonable misunderstanding of how our translation system works. It would be great to add some sort of warning that points people either to transifex or to our translation documentation.

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Edit by @alya 2025-11:

Rather than a lint rule, we should have a GitHub action that sends the following message:

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Please don't edit translation files directly, as this does not work with Zulip's translation system. If you're trying to add an improve a translation, follow these guidelines to join Zulip's team of volunteer translators on Weblate. If you're a developer, see the internationalization for developers guide.