Setup a CI using Github Actions

[Mesabloo]

Taking the time to setup a CI for Zilch is something that must be thought about.

It has already been done in N⋆ and allows for:

• building on multiple OSes (e.g. Windows, MacOS, etc)
• running tests after each push to prevent breakage of the code base
• running some integrity checks when a PR is submitted to ensure code validity (it should at least compile)
• automatically building & releasing binaries when pushing a tag

Fortunately, Github Actions are powerful enough for this kind of work (please bear in mind the limit of 2,000 minutes/month for free organisations).
For now, because there is absolutely nothing in the master branch, this can be postponed.

---

Note: I'm not sure how this will be play with the “Zilch in Zilch” thing, trying to compile some Zilch code after the second version; but this will handled when Zilch reaches such state.