feature: add a --check option to sprocket format

[peterhuene]

This option would be useful in CI where it would:

• Format a WDL document or directory containing WDL documents
• Instead of overwriting the files, it would diff the format output with the original output and treat any difference as an error; it should print out a nice git-like diff of the changes (the pretty-assertions crate has a very nice implementation of this).
• It should exit with a non-zero status if any file had a diff.

[a-frantz]

How much of the logic for --check should be in this repo? I think that if the answer is "all of it", this is going to be the first case of "business logic" living in the sprocket repo. Not exactly a problem, but something that should be done intentionally.

I wonder if this would make more sense to live in one of the wdl- crates (though which one isn't obvious to me).

[peterhuene]

I think it'd be fine to expose the "check" functionality from wdl-format.

[a-frantz]

I've flipflopped. The "check" functionality isn't really anything other than input == formatted. I think it's fine to house that logic within sprocket just to keep things simple. Consider my comment revoked.