Feature request: default handling of undefined values in the standard lib json module

[erlend-axelsson]

Is your feature request related to a problem? Please describe.
I have a use case where I need to send a json object with an explicit undefined value, and I discovered that the default behavior of json:encode/1 and json:decode is to encode the atom 'undefined' as a string, and throw an exception while decoding.
While it isn't a massive issue I believe it would be a quality of life improvement to treat 'undefined' as a special case like how 'null' is.

Describe the solution you'd like
Add a special case where 'undefined' gets (en/de)coded as undefined like how 'null' gets (en/de)coded as null in the default json:encode and decode functions.

Describe alternatives you've considered
Alternatively providing an encoder and decoder like json:encode_key_value_list_checked/2 which handles 'undefined' as undefined would also work if changing the defaults risks introducing regressions in existing code.

[erlend-axelsson]

I read up on the RFC 8259 and ECMA 404 standards, so it seems I was wrong in believing undefined to be a part of the JSON standard.

I still think there is some value in at least allowing the option of a "lax" encoding/decoding since it's not terribly uncommon for JSON "in the wild" to contain undefined values. It was relatively easy to get json:encode/2 to encode undefined, however it seems that the decoding part is hard coded to reject undefined as decode/1, decode/3 and decode_start/3 all immediately call json:value/6 which then errors on the function overload at line 797.

ref encoding undefined:

custom_encode(undefined, _Encode) ->
    <<"undefined">>;
custom_encode(Value, Encode) ->
    json:encode_value(Value, Encode).
custom_encode(Value) -> json:encode(Value, fun custom_encode/2).