For `SyntaxCollection`s with elements that conform to `HasTrailingComma` add `append` function that ensures the previous element has a trailing comma

Create an extension of `SyntaxCollection` if `Element: HasTrailingComma` that adds `append` methods, which make sure that any two elements are separated by a comma. That would be useful to e.g. add a new element to an `ArrayExprSyntax`