feature: add ScalarMarshaler and ScalarUnmarshaler interfaces

[x448]

ScalarMarshaler and ScalarUnmarshaler interfaces were added to the encoding package in golang/go:

• https://go-review.googlesource.com/c/go/+/553175
• JSON: https://go-review.googlesource.com/c/go/+/553176
• GOB: https://go-review.googlesource.com/c/go/+/569276
• XML: https://go-review.googlesource.com/c/go/+/568896

Description from https://go-review.googlesource.com/c/go/+/553176

encoding/json: handle encoding.ScalarMarshaler and encoding.ScalarUnmarshaler

Per discussion on https://go.dev/issue/56235, Marshal checks for ScalarMarshaler strictly
after TextMarshaler, and Unmarshal delegates to TextUnmarshaler or
ScalarUnmarshaler according to whether the corresponding JSON value is a
string, Boolean, or number. Neither function handles a complex128 type
argument to the interfaces.

[fxamacker]

Thanks for opening this issue. I will look into it.