isFixedFeature function
Whether featureKey is FIXED (non-overridable) at edition — i.e. setting
it via a features option is a hard error.
Implementation
bool isFixedFeature(int edition, String featureKey) {
final entry = _defaultsEntryFor(edition);
final fixed = entry['fixed'] as Map<String, String>;
return fixed.containsKey(featureKey);
}