Trailing Commas in Matrices

In the parser a matrix with trailing commas (final commas) has the following error
```py
[1, 2, ]         # yields SyntaxError: Value expected (char 8)
```
But objects with trailing commas works fine
```py
{a:1, b:2, }.    # yields {"a": 1, "b": 2}
```

It seems like a useful feature according to:

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Trailing_commas