Add "separator" to parse nested structs about envy HOT 2 OPEN

serde_with::with_prefix!(pdatabase "database__");
#[derive(Debug, serde::Deserialize)]
struct Config {
    #[serde(flatten, with = "pdatabase")]
    database: Database,
}
#[derive(Debug, serde::Deserialize)]
struct Database {
    name: String,
}

fn main() {
    std::env::set_var("DATABASE__NAME", "foobar");
    let config: Config = envy::from_env().unwrap();
    dbg!(config);
}