Add support for externally tagged unions

[sphinxc0re]

Externally tagged unions it the default for serde enums in e.g. JSON.

An enum like this:

pub enum Field {
    Floor,
    Wall,
    Portal { id: usize, target: MapAddress },
    Exit,
    Special { id: usize },
}

would convert to this:

export type Field =
  | "Floor"
  | "Wall"
  | {
      Portal: {
        id: number;
        target: MapAddress;
      };
    }
  | "Exit"
  | {
      Special: {
        id: number;
      };
    };

It looks kinda ugly but it's very useful to me and it's the serde default.

[NyxCode]

I have thought about this, and I do agree!
The default serde behavious looks disgusting, but we need to be compatible to it.
I've created this new branch to make this the default behaviour.

[NyxCode]

closing in favor of #38