Prettier style trailing commas

[dawust]

This pull request addresses the issue described in Issue #668 and Issue #1285 .
A flag UsePrettierStyleTrailingCommas is introduced which ensures a trailing comma for anonymous object/collection/initializer/switch expressions, list patterns and enum declarations in the case a line break occurs.

I hope the implementation of the feature isn't too invasive. Obviously it would be easier to implement the feature non-configurable.

I'm not sure how to deal with the tests, for now I enabled the flag and adjusted the corresponding tests.

Thanks for any feedback!

[belav]

Looking good so far! Thanks for taking this on.

The one big piece missing, is making sure SyntaxNodeComparer doesn't report the added/removed comma as a warning along with adding some tests in SyntaxNodeComparerTests.

I'm hoping the logic can go into one of the methods in SyntaxNodeComparer that compares lists.

[dawust]

I'm hoping the logic can go into one of the methods in SyntaxNodeComparer that compares lists.

I guess it's ok to just omit the last separator (expression count - 1) when comparing the separators of the original node and the formatted node!?

[belav]

I guess it's ok to just omit the last separator (expression count - 1) when comparing the separators of the original node and the formatted node!?

Yeah that seems good enough, I don't see the csharpier code somehow being so screwed up that it replaces the last comma with a different separator.... and if it did then I don't think we could actually generate a syntax tree from the result.

[belav]

Nice work on all the tests, thanks for the contribution!