Add CI and swift-format config

[GNMoseke]

Depends on #1

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]

Looks good. I tend to agree to those initial formatting rules. Fine to change some if we ever run into anything particular of course.

Might have to disable the macOS tests for now since they will either make the job fail or won't do anything since everything is commented out by #ifdef canImport(_Differentiation)

[JaapWijnen]

Also let's copy this config over to the other repos once we have agreed on these and this is merged

[tmcdonell]

• I don't have enough experience writing swift to be opinionated on style (except no hard tabs, ever), so happy to go with your recommendations
• I agreed having -Werror from the start is a good idea
• We should be able the make a template repository so we don't have to manually copy and paste to each new repo

[JaapWijnen]

I'd like to add this rule in a next PR any thoughts @GNMoseke?
NoAccessLevelOnExtensionDeclaration I really dislike not being able to read the access level at the location of the definition.
How do we want to keep these settings in sync across repos? We already have 4 repos where formatting is relevant atm.

• swift-differentiation
• swift-differentiation-testing (in a sub folder)
• swift-numerics-differentaible
• swift-collections-differentiable

Or do we do this by hand for now?