Type check the js files that have JSDocs

[canova]

We have files like bin/l10n-sync.js that have JSDocs in them, but apparently we are not checking them with yarn ts correctly. See my comment here: #5659 (comment)

It would be good to properly check them with typescript. To be able to do that I believe we need to add "allowJs": true to tsconfig.json, and add // @ts-check to these files.

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