std.json: Fixed issue 12969. opIndexAssign operator for JSONValue added....

... Changed order of elements in enum JSON_TYPE so NULL is a default type when JSONValue is not initialized

Signed-off-by: Uranuz <neuranuz@gmail.com>
This can break any code that has been relying on specific numeric values and / or ordering of `JSON_TYPE`.
Instead you can keep existing enum definition but add default initializer to type_tag field:
```D
    private JSON_TYPE type_tag = JSON_TYPE.NULL;
```