Test in CI that we're in sync with problem-specifications

...but we don't want our CI to break by upstream changes.

So we pin a version of problem-specifications as a submodule. The updating workflow would then be: Pull the submodule, run the test generator over all implemented exercises, fix what's broken (hopefully all caught by CI), and open a PR.

This is blocked by `configlet` being able to use a pinned version of problem-specifications, see https://github.com/exercism/configlet/issues/816.

Probably in a later step, we could add a CI cron job to check for upstream changes and open an issue (or even a PR with the update already done!)