Better error messages with `,`

[QuickWrite]

When the JSON parser parses a JSON document like this

{
    "test": 42,
}

then it should throw an error because there is a comma that resembles another entry.

And the parser does exactly that, but it shows that the } is the error even though the
comma is clearly the error.

---

This is also the exact same thing that happens with arrays:

[
    42,
]