Provide a way to detect what extensions are enabled via TH
This would be helpful for providing users with intuitive/explanatory error messages if generated code relies on an extension that the user may not have enabled.
Sample specification, to get things started:
enabledExts :: Q [Extension] isExtEnabled :: Extension -> Q Bool data Extension = LiberalTypeSynonyms | RankNTypes | ... -- mirroring ExtensionFlag in DynFlags
See #10819 (closed) for an example case where this could be of use.