Capture output of eval plugin and some girls scout changes

[dschrempf]

This PR contains some girl-scout changes as well as changes to how the eval plugin treats stdout and stderr.

Eval plugin

We now capture output to stdout and stderr induced as a possible side effect by the statement being evaluated. This is fragile because the output may be scrambled in a concurrent setting when HLS is writing to one of these file handles from a different thread.

Fixes #1977, most likely also #4390 (although this is debatable, we could do something more sophisticated).

-- >>> putStrLn "Hello"
-- Hello

-- >>> hPutStrLn stderr "Hello"
-- Hello

-- >>> putStrLn "Hello," >> pure "World!"
-- Hello,
-- "World!"

Girl scout changes

• Docs: Fix some links in eval plugin in-source documentation
• Clarify documentation of expected test failure
• Update Nix Flake (Darwin issues seem to be solved)
• Update some tests and avoid mentioning GHC 9.2
• Remove comment referring to GHC 9.2 and fix code

[fendor]

Needs a proper test in hls-eval-plugin, but otherwise looks good to me, thank you for the clean up! Feel free to merge once you've added a test :)

[fendor]

Slightly weird, probably needs to be fixed one way or another?

[dschrempf]

I didn't realize the eval plugin changed some other results. Thanks for picking that up. I will investigate.

[dschrempf]

I fixed those WAS NOW errors, one question though:

Does the plugin also evaluate all actions in the same comment block in your case?

If I evaluate one action in this comment block, all of them get evaluated and a lot of WAS NOW messages get added. Some of them are wrong. I am not sure if this PR added this discrepancy, or if the problem was already there before. I will check.