Replace Makefile with npm scripts

[josebolos]

Github Actions will also need to be updated accordingly.

We can probably use pa11y/pa11y#551 for inspiration.

[danyalaytekin]

I've done this for a couple of repos now, and it has definitely made sense for projects which didn't have more complex testing setups (such as pa11y-lint-config), but perhaps less so for pa11y-webservice, where there are integration tests that require some repetitive fiddling which could be encapsulated into a script. I'm not sure that those kinds of compound scripts are an npm package level-concern, even if we made an npm script defer straight to a shell script. Some people smuggle this kind of stuff into a Dockerfile, but we're not using Docker. I'm getting somewhere with nektos/act, so perhaps that can take care of this problem, since we can just run the same workflows as we have in GitHub Actions. In any case the connection to the more generalised makefiles will be removed whichever way we go, since they've been marked as obsolete.