Add check_unused_messages in tidy

[chenyukang]

From #115728 (comment)
The check is not 100% accurate, I guess it's enough for now.

[rustbot]

r? @Mark-Simulacrum

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[Mark-Simulacrum]

r? @davidtwco perhaps -- want to check on:

• Is there some risk with deleting ftl strings, e.g., for translations?
• Can we rely on some other tooling to manage unused detection for us? Feels like the heuristic here is a bit rough vs. potentially something built in to the compiler or so.

[Mark-Simulacrum]

This feels like it might get quite slow. It seems like we're scanning ~every Rust file in the same compiler crate for each fluent string. But, maybe OK for now, I don't think we need to block on it at this time.

[davidtwco]

I think something like this is worth doing, but this does seem like an expensive check. I've had some ideas for how we can do a bunch of our Fluent infrastructure better and it would solve this, I've pinged @chenyukang on Zulip about this to see if they're interested in working on that.

[chenyukang]

Thanks @davidtwco , I will take a look at your thoughts.
I removed the tidy check and kept only the cleanup in this PR.

[davidtwco]

@bors r+

[bors]

📌 Commit 42a033a has been approved by davidtwco

It is now in the queue for this repository.