Disallow Trailing Commas in dub.json

[nordlow]

As trailing commas are not accepted by the standard then maybe DUB shouldn't accept it either:

https://stackoverflow.com/questions/201782/can-you-use-a-trailing-comma-in-a-json-object

This can be confusing to D newcomers who want to use Emacs for their D development. The error occurs in Emacs' standard library so its very hard to understand what is wrong.

See also discussion at atilaneves/flycheck-dmd-dub#5

[p0nce]

I agree. As practical as trailing commas are, JSON + trailing commas is not exactly JSON.

[s-ludwig]

Agreed, it should go through a regular deprecation phase and first warn about it for maybe a year or two, so that existing packages don't get broken.

[nordlow]

Great. Thanks, for now.

[wilzbach]

Closing this as a duplicate of #1205 (while GitHub allows to mark this, I don't see any reason in keeping two issues open for the same goal)