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