Add support for JSON serialization with System.Text.Json

[thomaslevesque]

Using System.Text.Json, we can serialize Option<T> values to this:

{
  "Value": 42,
  "IsSome": true,
  "IsNone": false
}

but not deserialize it (it returns Option<T>.None).

The library should provide be a converter.

The JSON format could be something like this:

// Some
{
  "Value": 42
}

// None
{}

Note: serialization with Newtonsoft.Json works as expected (with the IsSome/IsNone properties)