[SR-12484] Make trailing comma behavior configurable (again)

[allevato]

Previous ID	SR-12484
Radar	rdar://problem/62201593
Original Reporter	@allevato
Type	Bug

Additional Detail from JIRA

Votes	2
Component/s	swift-format
Labels	Bug
Assignee	None
Priority	Medium

md5: 87a114148682a4a2db0f13538dd1e622

Issue Description:

When we migrated trailing comma correction to the pretty-printer (#120), we unintentionally lost the ability to disable it.

We should make it configurable again and consider a tri-state: do nothing (keep whatever the user wrote), always remove trailing commas, or always add trailing commas.

[beccadax]

@swift-ci create

[swift-ci]

Comment by Jeremy Saklad (JIRA)

I think it is also important to consider the primary purpose of trailing commas: making version control diffing cleaner.

I think the default behavior should be thus:

1. Trailing commas are not used when an array or dictionary literal has one line

2. Trailing commas are always used when an array or dictionary literal spans multiple lines with one item per line

3. Array and dictionary literals that span multiple lines have one item per line

That’s just my personal opinion, of course. Each of those rules should be independently configurable.