Add support for "warnings as errors"

[Kyle-Ye (Kyle)]

@ktoso

Please make sure how it works with transitive dependencies. We fixed the behavior for warnings as errors in SwiftPM recently so it affects your package, and not packages you depend on - since you have no chance of fixing them after all. The same should be true for this option, make "my" warnings errors, but if I depend on e.g. package X and it has a warning for some reason, this need not break my build.

Yes, I think other packages' documentation warning should not affect the main target. I think we could either just suppress all the diagnostic from other packages or the warnings-as-errors transform should only work for the main target.

If we choose the former, it will be another PR which does not block the current discussed PR.