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`