Support RFC #71 (trailing commas)

[patjakdev]

https://github.com/cedar-policy/rfcs/blob/main/text/0071-trailing-commas.md

[gregory-nisbet]

This issue is not closed yet; sorry for the confusion.

https://github.com/cedar-policy/rfcs/blob/main/text/0071-trailing-commas.md mentions a few places where trailing commas are now allowed.

1. scopes
2. sets
3. records
4. all function calls

My PR only addresses (1). The others are still outstanding.

[philhassey]

@gregory-nisbet Thanks, I've re-opened it!