Support standard syntax for language pragmas in GHCi

It would be convenient to be able to use the standard (module level) syntax for language pragmas in GHCi

{-# LANGUAGE Extension #-}

instead of just the GHCi specific syntax

:set -XExtension

For example, this would simplify copy-pasting examples and code into GHCi, especially now that most other module level syntax is supported. It might also reduce confusion for new users, since the syntax is the same in both cases.

If it is not too difficult, I might take on this myself as a good first bug.

Trac metadata

Trac field	Value
Version	8.0.1
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	GHCi
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