Allow trailing commas in optionaValues literal

It would be nice to be able to leave trailing commas in a `optionaValues` literal format. I'm not sure the best way to change the grammar, but a possible idea could be:

```
optionValues = exprNode
| '[' , valueList [, ','] , ']' ;
```

This may have a conflict with `valueList` since it also contains commas, but I think it will work fine, since when in `valueList` all infix commas will be consumed, leaving just the trailing comma to be optionally consumed by `optionValues`

Start by locating the grammar productions for optionValues and valueList, then check how commas are currently parsed in optionValues literals. Confirm that a trailing comma is accepted without changing infix comma handling, and add or update grammar tests if the project provides them.

Written by the indexing model from the issue text.