Build and check formatting using Github Actions

[oscar-b]

An initial workflow which will build and ch;eck code formatting.

The formatting change of package.json is done so that changes from yarn doesn't change the indentation all the time.

[oscar-b]

This is what it looks like in my repo, I assume it's not enabled here automatically without being merged first.

https://github.com/oscar-b/de1/commit/9173711bd7b0425d628203b34610be6d4d3a0fd5/checks?check_suite_id=387678709

[oscar-b]

@LukasBombach Could move the prettier settings to package.json if you prefer that.

[LukasBombach]

Nono, I'm actually in favor of separate files because the package.json might blow up at some point, I was just trying to escape formatting-discussion-hell (and a little bit the tabs-vs-spaces) discussion by just leaving the choices to someone else 😱 😆

[LukasBombach]

😍 great work!