query

Queries a value by JSON path expression.

Supports `$.key`, `$.key[index]`, and nested paths like `$.users[0].address.city`.

Example:

    var city = Json.query (root, "$.users[0].address.city");

Parameters:

root	the root value.
path	the JSON path expression starting with `$`.

Returns:

the value at the path or null if not found.