Support untagged variant

[cshaa]

Serde supports untagged enum variants (added in this PR):

#[derive(Serialize, Deserialize)]
#[serde(rename_all = "kebab-case")]
pub enum Foo {
  Bar,
  Qux,
  #[serde(untagged)]
  Other(f64),
}

This should result in the following TS type:

export type Foo = "bar" | "qux" | number;

However, if you add Tsify to the derive clause, you get an error instead:

unknown serde variant attribute `untagged`

[Enduriel]

This doesn't produce an error any more, instead now it just seems to be ignored (which is still not ideal)

[madonoharu]

Here is the output in the current version.

export type Foo = "bar" | "qux" | { other: number };

Further improvements are still needed.