Bump version to GHC 9.12?

[pe200012]

Hi, thank you for making such a great tool!

I notice that higher version of GHC contains some newer extensions (e.g. multiline strings) that break retrie. To experiment, I forked the repository and successfully upgraded it to work with a newer GHC, and I would now like to contribute these changes upstream.

However, I am not sure what should be done exactly. Particularly, I’ve seen significant changes in core AST types. Should I add macros to keep compatibility to old versions? Any guidance on test coverage or release policy for dropping old GHC compatibility would also be very helpful.

Thanks for your time and for building retrie!

[pe200012]

Oops, I’ve just realised that there’s already an open PR working on upgrading to GHC 9.12. Maybe I should close this issue.

Sorry for disturbance.

[pe200012]

I made some modification based the stale PR. #9