Function is_builtin_function 

pub fn is_builtin_function(name: &str) -> bool

Whether name deserializes to a typed built-in variant at all.

Equivalent to builtin_function_kind(name).is_some(). This answers “is this a name the crate special-cases”, not “can this engine run it” — a BuiltinKind::RequiresHandler name returns true here whether or not a handler is registered.