Add CI and swift-format config

[GNMoseke]

This adds basic CI (linting and tests on macOS/ubuntu) for PRs. Some things to keep in mind:

• the .swift-format file here is opinionated based on my personal preferences, I am happy to tweak them based on other maintainers' thoughts
• By default I've set swift to build with -Werror as I think it's a good practice to get into off the starting block. Again, if we forsee unavoidable errors (@deprecated() 😡 ) being added, happy to remove those flags
• The macOS tests are running only against the latest stable Xcode - happy to dig into more custom options there if desired

[JaapWijnen]

Can we do the test ci in a separate PR from the formatting perhaps? Since we're still having some concerns around swift format capabilities.

[GNMoseke]

Can we do the test ci in a separate PR from the formatting perhaps? Since we're still having some concerns around swift format capabilities.

Sure, sorry have not had time recently to get back to this. Will try and split by monday.

[GNMoseke]

See differentiable-swift/swift-differentiation#6 (comment) - the same changes have been pushed here

bump @JaapWijnen @clackary