Derive Flat Debug

This macro generates a custom Debug implementation for enums that:

• Flattens single-field tuple variants when the variant name matches the field type name or when #[debug(flatten)] is used.
• Skip flattening variants with #[debug(skip)].

Restrictions:

• Only applicable to enums (not structs or unions)
• #[debug(skip)] and #[debug(flatten)] cannot be used together on the same variant

Example usage:

#[derive(DebugFlat, PartialEq)]
pub enum QueryToken {
    #[debug(flatten)] Group(Molecule),
    Molecule(Molecule),
    Atom(Atom),
    #[debug(skip)] Infix(Infix),
    #[debug(skip)] Postfix(Postfix),
    Filter(Filter),
}

#[derive(Debug, PartialEq)]
pub struct Molecule(pub Vec<QueryToken>);