Initial support for GHC 9.4

[kazu-yamamoto]

This PR would be better than #375 since doctest can be compiled with GHC 9.4.
Unfortunately, four test cases fail.
But I believe that this is a good start point for discussion.

I confirmed that GHC 9.0 and GHC 9.2 works well.

Cc: @parsonsmatt

[kazu-yamamoto]

@sol Would you take a look?

[sol]

Hi @kazu-yamamoto 👋 This looks great 👍

Before moving forward with this, I think we will need:

1. A separate PR to enable tests for GHC 9.0 and 9.2. If you want to look into this then you can use e.g. https://github.com/hspec/hspec/blob/main/.github/workflows/build.yml as a starting point.
2. Enable GHC 9.4 as part of this PR.

At the moment I am traveling, but I will try to reply here.

[kazu-yamamoto]

I agree that tests in CI should be deployed.
But I don't want to touch CI stuff of this project by myself.
Would someone take over this job?

[sol]

@kazu-yamamoto I updated CI, can you please rebase your changes on main and add 9.4.1/9.4.2 to the list of GHC versions at https://github.com/sol/doctest/blob/main/.github/workflows/build.yml#L52.

[eddiejessup]

I've tried to fix the tests; made a PR at #382 with some details

[sol]

Closing this in favor of #382.