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.

Edited Mar 10, 2019 by Ben Gamari

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