Add Nix flake for Cabal-based development environment

[yjymosheng]

Provide a Nix flake that sets up a GHC 9.2.8 development shell with cabal-install, HLS, and two convenience wrappers:

• cabal-build: runs cabal v2-build in the exercises/ directory
• cabal-test: runs cabal v2-exec runhaskell <file> in exercises/

This allows NixOS users to run exercises with Cabal without needing Stack. The existing Stack workflow is not affected.

Usage:
nix develop
cabal-build
cabal-test exercises/Set1Test.hs

[yjymosheng]

Some images are used to demonstrate that the intended use is normal and does not affect the normal use of non-NixOS stacks.

[yjymosheng]

While adjusting the PR, I saw some issues that already exist in the Issue ，so added a README to support the usage instructions for Cabal and Nix.

[yjymosheng]

Sorry for the additional review request. I just rebased and cleaned up the commit history. The original history was a bit messy, so out of a desire for cleanliness， I cleaned it up.

[abdivasiyev]

LGTM

[opqdonut]

This looks ok. I'll try to merge this soon. Merging needs a bit of extra work since the private repository that this repository is generated from needs to be updated to match.

[yjymosheng]

This looks ok. I'll try to merge this soon. Merging needs a bit of extra work since the private repository that this repository is generated from needs to be updated to match.看起来没问题。我会尽快合并。合并需要一些额外的工作，因为生成此仓库的私有仓库需要更新以与之匹配。

Got it.

[AfkaraLP]

You should maybe update the .gitignore to allow commiting the flake.lock file (!flake.lock) else direnv will keep trying to rewrite the flake lock file as it does not see it

[yjymosheng]

You should maybe update the .gitignore to allow commiting the flake.lock file (!flake.lock) else direnv will keep trying to rewrite the flake lock file as it does not see it您应该更新 .gitignore 文件，允许提交 flake.lock 文件（ !flake.lock ），否则 direnv 会一直尝试重写 flake lock 文件，因为它找不到该文件。

1. I wasn't sure about the "not found" scenario - direnv allow automatically generates flake.lock on first run, so there shouldn't be any issue.

2. Since nixos-24.05 is a stable branch with no updates for ~2 years, I didn't think reproducibility would be a practical concern.

3. My initial preference was to keep changes to the original setup minimal. But you're right that including the lock file is the better practice, so I've added it . However, it depends on the onwer's idea.

[yjymosheng]

Additional change : I also removed the shellHook that attempted to auto-run cabal update . It wasn't working reliably (likely due to my misunderstanding of cabal's behavior), so I replaced it with a note in the README instead. This also slightly improves shell startup time.

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+ Once inside the Nix shell, you can use the Cabal commands described above. **If you encounter issues with missing packages, try running `cabal update` first to refresh the Hackage index.**

If you encounter issues with missing packages, try running cabal update first to refresh the Hackage index.

The above is the main content that has been added.