add support for unit_variant

[Freax13]

This pr adds support for serializing enum unit variants with their corresponding names.
With this pr one no longer has to rely on strings for (de-)serializing enumerations.

Example:

#[derive(Serialize, Deserialize)]
pub enum Effect {
    #[serde(rename = "minecraft:overworld")]
    Overworld,
    #[serde(rename = "minecraft:the_nether")]
    Nether,
    #[serde(rename = "minecraft:the_end")]
    End,
}

(note that this example uses #[serde(rename = "...")] to rename the variants but this isn't necessarily always required)

[atheriel]

Looks good, thanks!

[Freax13]

would you mind updating the crate on crates.io?

[atheriel]

Should be visible shortly.

[Freax13]

thanks 👍