[Not Urgent, Stylistic] Add newlines in long function definitions in `src/YAML.jl`.

[Paalon]

[kescobo]

I really dislike this as well. I'd be open to a newline on kwargs or something for these (I showed an example above), but in that case, I think I'd want a function block. This kind of enormous signature strikes me as an abuse of of the short syntax anyway.