Added github actions CI.

[newAM]

Would there be any interest in adding CI to this repository?

[Ravenslofty]

Here's the question. How do you CI a USB stack? I'm all for testing and that kind of thing, but my gut instinct is that it would require physical hardware to check against, and that's going to be impossible on a CI virtual machine.

[dholroyd]

You are right @Ravenslofty, running the code to assert that it works is going to be tough.

My feeling is that there's still some value in seeing if the code still builds etc in the face of future PRs -- I'd support adding some actions.

[newAM]

I have done embedded CI before it is entirely possible, just expensive. Probably not feasible here.

Compile checks are always free though.