add Makefile targets for fix-whitespace (backport #10376)

[mergify]

make whitespace now runs fix-whitespace --check --verbose and make fix-whitespace runs fix-whitespace --verbose. You will need to install fix-whitespace yourself, but that's just a cabal install away.

This is another step in the direction of fixing #10263.

Template B: This PR does not modify behaviour or interface

E.g. the PR only touches documentation or tests, does refactorings, etc.

Include the following checklist in your PR:

• Patches conform to the coding conventions.
• Is this a PR that fixes CI? If so, it will need to be backported to older cabal release branches (ask maintainers for directions).

---

This is an automatic backport of pull request #10376 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of 2858dc0 has failed:

On branch mergify/bp/3.14/pr-10376
Your branch is up to date with 'origin/3.14'.

You are currently cherry-picking commit 2858dc099.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   CONTRIBUTING.md
	modified:   Makefile

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   .github/workflows/whitespace.yml

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally