Accept trailing commas wherever commas are used as a separator

[degory]

Anywhere where we require commas to separate items in some kind of list, we should accept (but not require) a trailing comma after the last element in the list.

• List literals
• Tuple literals
• Destructuring left-hand-side patterns
• Enum members
• List of variables in a let statement
• Function and method definition formal argument lists
• Function and method call actual argument lists