Add style guide check + preview of differences

[troibe]

Is your feature request related to a problem? Please describe.
I would like to use ormolu in a git action to check if the code complies with the style guide.

Describe the solution you'd like
Maybe I didn't find it in the documentation but is there a mode that just checks if the style guide is met and returns the parts of the code where there would be differences?

Describe alternatives you've considered
Of course I can just execute the formatter and then compare unformatted and formatted file but it would be nice to have this functionality build in.

[mrkkrp]

Indeed, this is something I'd like to have too. The suggested edits should be printed to stderr in check mode, right now we just return non-zero exit code if the inputs are not already formatted.

[mrkkrp]

Duplicate of #656.