Add Haddock hyperlinks to hover for type holes

I would find it very helpful if the hover information could include links to Haddock documentation (if documentation is available).

A common situation when working with an unfamiliar library is needing to pass an argument to a function say `foo`, and a typical workflow would be to start writing:

```haskell
where
x = foo a b _notsure ( d e )
```

then hover over `_notsure` to find its expected type. Ideally then I'd be able to see the type needed, and be able to click on it for the documentation. For instance:


I'd like to be able to open up this documentation:


Having taken a quick look at how hover information is created, I don't see how to simultaneously include hyperlinks and have syntax highlighting, as the current hover information is typeset by encoding it in a Haskell markdown codeblock.

Currently, hover information is available for set functions, e.g.:


Something similar could be done in the previous situation, including a link to `WindowHints`, but that still seems a bit limited to me as the type might be something more complex and I might want to click on some component of it. It's especially annoying as if I click on the documentation for `windowSetGeometryHints` in the previous example, I can't then click on `Geometry` or `WindowHints` to get to that part of the documentation because of [`ghcide` !61](https://github.com/haskell/ghcide/issues/61), as those are part of the `gi-gdk` package (whereas `windowSetGeometryHints` comes from the `gi-gtk` package).