Add CI to this Repository to Ensure the Contained Files are Compiled Slice

[InsertCreativityHere]

We should add CI to this repository which runs the contained Slice files through each of our supported compilers.

Originally, I was thinking that just using the slicec binary would be sufficient, but this cannot validate language specific attributes. So, if it's not a terrible pain, the most correct approach would be to run these files through slicec-cs (and eventually slicec-rs) to check for any diagnostics.

I don't view "CI efficiency" as important for this repository, since it will very rarely see any changes.

[pepone]

Adding cross repository CI steps is a big pain. I think is fine if attributes are validated in the compiler repositories, we should add the attributes in the language specific repositories, and only push here after the PR is clean.

[InsertCreativityHere]

I'm not saying that we should rebuild slicec-cs from source, but couldn't an action download and run slicec-cs against the files?

[externl]

This was added with #28