Allow trailing commas in parameter lists

[Ben_Cohen]

@supervillain

In the spirit of simplifying the syntax, in the same way that the semi-colon was removed as a line terminator, it could also be "nice" to just omit the comma as a separator for multi-line parameters, including arrays and dictionaries.

This is SE-0257 which was returned for revision due to some (unfortunately fairly tough to overcome) parsing ambiguities. Personally, I agree it would be really nice.

[mesqueeb]

I just want to add my vote to allowing trailing commas in the same places as JavaScript allows them. In the spirit of attracting new developers to the Swift ecosystem with the least friction possible.

Also, it's not because you can add trailing comma's in function parameters, that you are forced to. So why not allow it for the rest of us who want it. : )