SE-0439: Allow trailing comma in comma-separated lists

[bob]

@allevato

As this proposal relates to formatting, I think it would be straightforward to extend swift-format's current handling of trailing commas in array literals (but without the one-element exception) to enforce trailing commas at the ends of lines anywhere that the proposal supports it, and to remove them otherwise. For example, if the trailing comma be

I would also consider the way Python’s Black formatter handles trailing commas: where possible, it interprets the presence or absence of a trailing comma as the user’s desire to have the list on multiple or one line(s) respectively, max line width permitting. Deleting a trailing comma and pressing “format” will put a list entirely on one line if possible; adding a trailing comma will break the list into one line per element.