Introduce trailingCommasInMultilineLists configuration

[TTOzzi]

Resolve #946

Added a configuration to support the use of trailing commas extended by SE-0439.

This configuration supports three values:

• always: always insert trailing commas in multiline lists
• never: always remove trailing commas
• ignore: preserve existing commas as-is

The default value is ignore.

The main concern was how to integrate this with the existing multiElementCollectionTrailingCommas configuration.
For now, trailingCommasInMultilineLists takes higher precedence, but to minimize its impact on multiElementCollectionTrailingCommas, it is implemented so that when set to the default value ignore, the behavior of multiElementCollectionTrailingCommas is preserved.

I’d appreciate any suggestions or feedback you may have.

[allevato]

Is there an abstraction we can come up to reduce the repetition in the individual visitors? Something like a helper function that accepts a generic parameter constrained to a collection of WithTrailingCommaSyntax elements and does the actual work.

[TTOzzi]

Is there an abstraction we can come up to reduce the repetition in the individual visitors? Something like a helper function that accepts a generic parameter constrained to a collection of WithTrailingCommaSyntax elements and does the actual work.

I was struggling because the point where a comma needs to be inserted differs for each syntax, but as I came to know about WithTrailingCommaSyntax, I came up with an idea!
I defined an abstract type and refactored the code accordingly. Thanks!

[allevato]

This could just be if shouldHandleCommaDelimitedRegion == true. Being written twice made me have to reread it a couple times to make sure they were referring to the same thing.

[allevato]

Hate to be nitpicky, but can you change this to isCollectionLiteral? isCollection feels like it could be talking about the node itself (which are all Collections).

Or... does this have to be a protocol requirement at all? It looks like you could just pass it in as another parameter to markCommaDelimitedRegion, since it has to be called individually for each node type anyway. That might be cleaner (but the rename is still valid elsewhere that it's referenced in the pretty printer).

[allevato]

Looking at this more closely, this feels too similar to the name of the other property below. I don't know if multilineLists really communicates the bit about parameter lists, tuples, closure captures, etc.

What about multilineTrailingCommaBehavior? Then, let's rename the enum to MultilineTrailingCommaBehavior and have the cases be alwaysUsed, neverUsed, and keptAsWritten. I think that helps to better document what's going on.

Later on (not in this PR), we could add a fourth case: onlyCollectionLiterals, which could take over the behavior of the other Boolean setting, and deprecate that one.