Highlighting for haddocks

As per the [haddock documentation](https://haskell-haddock.readthedocs.io/en/latest/markup.html), Haskell code can appear in Haddock comments in the following ways:

```haskell
-- code blocks:
-- @
-- f :: a -> a
-- f a = a
-- @
--
-- bird tracks:
-- > f 12
--
-- REPL examples:
-- >>> f 12
```

There are also delimiters for LaTeX: ` \( ... \)` (inline equation) and ` \[ ... \]` (displayed equation).

It would be nice if highlighting could be enabled for those cases.

Some other extra highlighting that could be done:

- headings with `=` `==` `===`, ...
- named chunks `$chunkname`
- `*bold*` and `/italics/`
- `@since` annotations
- URLs (VS Code automatically makes URLs clickable)

I think we could have a special haddock grammar file that we could include in the main Haskell grammar for Haddock comments.