Update HLS version and add GitHub action

[palas]

Changelog

- description: |
    Update GHC in Nix shell and HLS, and add GitHub action to verify HLS
# uncomment types applicable to the change:
  type:
  - test           # fixes/modifies tests
  - maintenance    # not directly related to the code

Context

This PR updates HLS to the latest version and ensures that it works on the repo by adding a GitHub action that checks.

How to trust this PR

Probably best to just test that it works for you.

Checklist

• Commit sequence broadly makes sense and commits have useful messages
• New tests are added if needed and existing tests are updated. See Running tests for more details
• Self-reviewed the diff

[smelc]

I confirm that HLS works for me on this PR 👍

Well done @palas 👏