Improve support for JSON casts

Currently, casting to and from std::json is supported only for scalar types. This proposes to extend JSON casting support to collection types as well as allowing to cast objects to std::json.

New rules:

1. <json>Object returns a json value containing the output representation of the object. That is, the result is the same as SELECT Object in JSON mode, including the type shape.

2. Scalar casts now work only for scalar JSON values, so casting a non-scalar JSON value to std::str is an error, and <json>'{"foo":"bar"}' returns a JSON string ("{\"foo\":\"bar\"}"), not a JSON object.

3. Non-scalar string-to-JSON and JSON-to-string casting is accomplished by the new str_to_json(str) -> json and json_to_str(json) -> str functions correspondingly.

4. <json>array_value returns a JSON array

5. <array<type>>json_value returns an EdgeQL array of the given type. null values in input produce an error (if the target type is not json, use json_array_unpack for fine-grained casting).

6. <json>tuple_value returns a JSON array and conversely <tuple<...>>json_value converts a JSON array into a tuple. null elements in input produce an error (if the target element type is not json).

7. <json>named_tuple_value returns a JSON object and conversely <tuple<name: type...>>json_value converts a JSON object/array into a named tuple. null values in input produce an error (if the target type is not json).

[msullivan]

I think this is done

[elprans]

Remaining work items in this issue:

• allow casting json to anyenum and vice versa (should be similar to the std::str cast in enumoperators.edgeql)
• allow casting json to tuple types
• allow casting json to array types