register method
Register or override the handler for function (engine-unaware closure).
Safe to call before or after engine construction. Pre-construction calls take precedence over built-in defaults; init will not overwrite them.
Implementation
void register(String function, FutureOr<Object?> Function(Object?) handler) {
_tombstones.remove(function);
_composedDispatch.remove(function);
_dispatch[function] = handler;
}