[#151] Add option to deserialize both null and missing as None

[Tarmil]

Add optional argument deserializeNullAsNone: bool to option .WithSkippableOptionFields(SkippableOptionFields).

let options =
    JsonFSharpOptions.Default()
        .WithSkippableOptionFields(SkippableOptionFields.Always, deserializeNullAsNone = true)
        .ToJsonSerializerOptions()

JsonSerializer.Deserialize<{| x: int option|}>("""{}""", options)
// --> {| x = None |}

JsonSerializer.Deserialize<{| x: int option|}>("""{"x":null}""", options)
// --> {| x = None |}

Compare with the current, and now default false behavior:

let options =
    JsonFSharpOptions.Default()
        .WithSkippableOptionFields(SkippableOptionFields.Always)
        .ToJsonSerializerOptions()

JsonSerializer.Deserialize<{| x: int option|}>("""{}""", options)
// --> {| x = None |}

JsonSerializer.Deserialize<{| x: int option|}>("""{"x":null}""", options)
// JsonException: The JSON value could not be converted to System.Int32.

Fixes #151, #176, #182.

Note: I have changed my mind since this comment, in that this setting only affect option and voption, but not Skippable. The reason is that Skippable exists precisely for fields that must be skipped, and so that Skippable<T option> can be used to explicitly distinguish between missing and null. This ability should be usable alongside option values that can be deserialized from either null or missing.