Add support for displaying :info

[jacg]

Another useful feature I miss from dante / intero.

Like haskell/ghcide#70, this might be difficult to implement, because it might not fit within the constraints of LSP?

[Avi-D-coder]

Short of full :info just showing constraints on hover would be useful.

[jneira]

afaik :info could be obtained indirectly in hls via the eval plugin like :type: (see #361)
In any case i think this should be a plugin in hls, so i will tranfer this if nobody complains

[ndmitchell]

Agreed, please transfer.