Add CI

[bjoernQ]

This adds CI.

I tried something new here: Using a justfile to run the actual checks and use as-little-as-possible of GitHub Actions

• good: this makes it really easy to do the same checks locally (just use just)
• downside: it's harder to see at a glance what exactly failed
• downside: if the job fails, rerunning means rerunning everything

I'd like to experiment with this a bit since the first point is really something I'd like to have - maybe there are even solutions for the other two points

[bjoernQ]

I don't know anything about just personally so can't comment on the justfile. CI config looks fine otherwise though.

It's also the first time I used it but I've seen it used for other Rust projects in CI and I wanted to give it a try

Thanks for the review!