Function register_whatsit 

pub fn register_whatsit<E: TeXEngine>(
    engine: &mut E,
    name: &'static str,
    get: fn(&mut EngineReferences<'_, E::Types>, <E::Types as EngineTypes>::Token) -> TeXResult<Option<Box<WhatsitFunction<E::Types>>>, E::Types>,
    immediate: fn(&mut EngineReferences<'_, E::Types>, <E::Types as EngineTypes>::Token) -> TeXResult<(), E::Types>,
    the: Option<fn(&mut EngineReferences<'_, E::Types>, <E::Types as EngineTypes>::Token) -> TeXResult<Vec<<E::Types as EngineTypes>::Token>, E::Types>>,
)

Creates a new “Whatsit” primitive and registers it with the engine.