[Improvement]: Improvements to JSON to record conversion

[MaryamZi]

Description

$title.

Describe your problem(s)

For

{
    "a": 1, 
    "b": "str",
    "c": null
}

we generate

type NewRecord record {
    int a;
    string b;
    anydata c;
};

Describe your solution(s)

At the moment,

1. when there are null values, the relevant field is generated as anydata, why not use json?
2. the generated records are inclusive record typedescs, open by anydata. Can't we generate exclusive record typedescs with json as the rest descriptor instead?

This way, the generated record will also be a subtype of json.

type NewRecord record {|
    int a;
    string b;
    json c;
    json...;
|};

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