CI: add hlint workflow

[Anton-Latukha]

As I understand, CI installs executable & runs it.

HLint gives the same output regardless of the GHC versions, so there is no reason to run it inside GHC matrix. In fact HLint executable installed does not need GHC at all.

[Anton-Latukha]

[skip circleci]

[Anton-Latukha]

It is a great moment to do a workflow change in HLint, because currently there are no hints, because all of them were cleared in #2523 which just merged. (& note that I reproduce the scope that old fmt.sh checks in.

For example, after syncing GHC 9.2 PR to master - inside PR there is also: https://github.com/haskell/haskell-language-server/runs/4629194854?check_suite_focus=true no hints under the old way of running workflow.

[Anton-Latukha]

Here I specifically produced a Lint suggestion:

So currently in all controlled files HLint is clean & HLint would show suggestions only in the files that PR chages.

[Anton-Latukha]

Kept this open to show that the workflow works.

[pepeiborra]

I like this. Will wait for @jneira to share his thoughts and approve.

Also need to add the new workflow to the GitHub PR requirements.

[Anton-Latukha]

Of course with availability to use different levels of messaging, the levels for hints can be tuned.

[jneira]

great, I like see this in another workflow, as it can be setup and run independently,
thanks!

[jneira]

hlint workflow added to required checks to merge

[Anton-Latukha]

We not see warning suggestions because #2523 covered all warning suggestions which current HLint setup monitors.

Workflow blocks on error severity just as before it was blocking inside test, but now it also presents warning level messages.

It is the same setup, but now information messages are not hidden inside build, but shown in review. Which motivates people to follow even soft linter suggestions.