Change defaults for TrainingComma hash/array cops

[skatkov]

I think that defaults for Trailing Commas cops should be switched to EnforcedStyleForMultiline: comma.

It helps with consistency, preventing copy-paste errors, and maintaining attribution (minimizing impact on diffs).

Quoting from /r/javascript: https://www.reddit.com/r/javascript/comments/6vqsjh/whats_with_the_trailing_commas/

Here are examples from rubocop:
https://docs.rubocop.org/rubocop/1.0/cops_style.html#styletrailingcommainarrayliteral
https://docs.rubocop.org/rubocop/1.0/cops_style.html#enforcedstyleformultiline-comma-3

[searls]

Sorry, we're keeping this no_comma. We'd initially set Standard up this way for about a year, but it was easily the most-complained-about setting. It also caused some obscure bugs & inconsistencies in edge cases around things like trailing commas in keyword args and hash parameters.

[sdhull]

Can you elaborate a bit on these "obscure bugs & inconsistencies in edge cases"? The rationale for enforcing trailing commas in multiline literals largely echos the rationale put forth for enforcing leading dots in multiline method chains. I don't have time to put together a sweet presentation like the linked issue but since this was the original setting I'm guessing some maintainers see the usefulness.

I'll just leave you with a quote from that thread

The git churn should be taken seriously here.
@strzibny

[sdhull]

trailing commas in keyword args and hash parameters

@searls I think I finally understand the point here. So what about setting preference for trailing commas for hash literals but no trailing comma for method invocations? (This is what we had set up at my previous company, which felt like a good balance.)

[ElMassimo]

Is there any advantage to not allowing trailing commas?

There are certainly advantages to having them in multi-line arrays and hashes.

[Timmitry]

This setting really prevents me from adopting Standard as formatter. Especially the git churn is an issue for me, as well as the extra work when wanting to switch two lines.