CI on GitHub Actions using a patched haskell-ci generated workflow

[andreasabel]

WIP, first successful run at https://github.com/andreasabel/alex/runs/2829343149?check_suite_focus=true

Rests on/includes PR #188.

[andreasabel]

This PR is now ready.

Please have a look at the supplied DEVELOPER.md for the explanation of the philosophy of this CI: It uses the workflow generated by haskell-ci and adds bits that are specific to Alex:

• additional dependencies: alex and happy
• generate Scan.hs and Parser.hs before doing sdist
• install the built Alex so that it can be found by the test runner (this seemed to be the easiest way, otherwise I have to somehow get reliably the path the build executable somewhere in dist-newstyle)

This PR should ideally squashed, but after merging #188 (so that #188's commits do not get squashed into the GHA specific stuff).

[Ericson2314]

I think I will just start with this one :).

Can you rebase this first? I think it would be good if the PR also didn't have the extra diff from #188, even if I do squash it, for anyone reading the PR after.

[andreasabel]

I rebased it onto current master, so now the #188 bits are gone. The commit sequence is otherwise unchanged and still reflects my try-and-error process, it is probably best to squash this into one commit.