HLS should show the module description on hover over an import

The module `GHC.Unit.Env` has a rich and descriptive module documentation, but there is right now no way to actually view it from HLS.

### Proposed Solution

On hover on the module declaration and hovering over an import, we should render the haddock module documentation.

I am not sure why we are not doing this yet already, but I think it should be reasonably straight forward.

Start by tracing HLS hover handling for module declarations and imports, then find how Haddock documentation is represented or rendered. The work is done when hovering either a module declaration or an import displays that module's documentation, including the content described for GHC.Unit.Env.

Written by the indexing model from the issue text.