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 DynFlagsSee #10819 (closed) for an example case where this could be of use.