[#676] Add back hlint and reuse checks to CI

Description

Problem: During !931 (merged) (merged) the 'hlint' and 'reuse lint' checks were accidentally removed alonside the trailing whitespace one. These aren't covered however and should be put back.

Solution: add back a step (each) for 'hlint' and 'reuse lint' in CI for MR validation.

Related issue(s)

Resolves #676 (closed)

✅ Checklist for your Merge Request

Related changes (conditional)

• Tests (see short guidelines)

  • If I added new functionality, I added tests covering it.
  • If I fixed a bug, I added a regression test to prevent the bug from silently reappearing again.

• Documentation

  • I checked whether I should update the docs and did so if necessary:
    • Root README and other README.md files
    • Haddock
    • docs/
  • I updated changelog files of all affected packages released to Hackage if my changes are externally visible.

Stylistic guide (mandatory)

• My commits comply with the following policy.
• My code complies with the style guide.