Trailing comma support for map, message literals

[TristonianJones]

Feature request checklist

• There are no issues that match the desired change
• The change is large enough it can't be addressed with a simple Pull Request
• If this is a bug, please file a Bug Report.

Change
List literals currently support a trailing comma; however, map and message literals do not. To make the language more consistent and easier to use with copy+paste for configuration objects, add support for a trailing comma to type construction literals.

Example

The following map construction should be supported:

{
   'a': 1,
   'b': 2,
}