Refactor the npx replacement

[lishaduck]

There wasn't really consensus on the original issue, but I wanted to write some code, so 🤷‍♂️
I improved the docs at least :)
For reference, the diff is -82 +82 lines, so it's literally a net neutral w/regard to maintainability.
It primarily does so by reusing existing code for emulating npx for elm-format instead of using another (hacky) handrolled solution I introduced in #197 (as a nice side effect, we test that the CLI works without --compiler in the unit tests again).

Closes #224.

[jfmengels]

Thanks!

PS/heads-up: leaving on vacation for 1-2 weeks. I'll likely see the PRs but I won't have a laptop.