Add json-file option

[9999years]

Adds a json-file option which consumes output from a previous hlint --json run.

[mihaimaruseac]

Can you explain what this PR aims to bring? What is the use case?

Also, it would be ideal if this PR focuses only on what it sets up to do. Right now, you also have changes on newlines escapes, which I think should be in a separate PR with its own justification

[9999years]

Sure! We use nix to package and run hlint, so there's not an hlint executable on the $PATH. We have hlint output JSON itself, and then we pass it through to this Action. Does that make sense? Happy to split the newline escape changes into another PR. (I'm picking this change up from the colleague that originally wrote it.)

[mihaimaruseac]

Sorry, I completely forgot about this. Yes please, let's split the newline change to another PR.

As a side note, I'm considering deprecating hlint-{setup,run} pair in favor of https://github.com/haskell-actions/hlint-scan but I need to make sure all current users can migrate without being impacted too severely. Can you please take a look and see if you can use hlint-scan instead? Thank you!