Command to dump type information for entire file

[lierdakil]

From atom-haskell/ide-haskell#133 (comment), this could be useful for, e.g. processing lhs into html to add pretty type signature popups. Besides, this could be useful for edge cases with frontends, e.g. to cache type information and show something in case ghc-mod fails for some reason (not necessarily a good idea, but it was suggested in one of @atom-haskell issues)

Creating this issue as to not forget about it entirely. Not necessarily suggesting we should run and implement this right now.

Marking as 'easy' since we already have that type information, just need to dump it, see atom-haskell/ide-haskell#133 (comment) for a bird's-eye overview (not necessarily accurate)