Json grammar doesn't allow array

https://github.com/epfl-dlab/transformers-CFG/blob/main/examples/grammars/json.ebnf#L6

I just realise that the grammar here for json only allows json object at root level.

In fact, json grammar accepts array and other atomic values like string or number at root level.

So a string itself is a valid json.

Here is the spec of JSON standard: https://www.json.org/json-en.html

Probably we just need to replace the root grammar rule `json::=object` to `json::= value` and fix the potentially impacted tests.