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!