Add intermediate representation

[brandonchinn178]

Adds a new completely optional (not even exported in import Commonmark) code path for people who want to inspect or transform the markdown content before rendering as Html (). Not sure if it's worth merging, but it was at least an interesting exercise. I'm putting up this PR mostly for demonstration purposes

Tested in GHCi:

ghci> import Commonmark.Nodes

ghci> commonmark "foo.md" "# hello\n```haskell\nx = 1\n```" :: Either ParseError (Nodes (Html ()))
Right (Nodes {unNodes = [Node {nodeType = NodeHeading 1 (Nodes {unNodes = [Node {nodeType = NodeStr "hello", nodeAttributes = [], nodeRange = Just foo.md@1:3-1:8}]}), nodeAttributes = [], nodeRange = Just foo.md@1:1-2:1},Node {nodeType = NodeCodeBlock "haskell" "x = 1\n", nodeAttributes = [], nodeRange = Just foo.md@2:1-5:1},Node {nodeType = NodeParagraph (Nodes {unNodes = []}), nodeAttributes = [], nodeRange = Just foo.md@4:4-5:1}]})

ghci> fromNodes <$> commonmark "foo.md" "# hello\n```haskell\nx = 1\n```" :: Either ParseError (Html ())
Right <h1>hello</h1>
<pre><code class="language-haskell">x = 1
</code></pre>
<p></p>

[brandonchinn178]

These types have an annoying type parameter because in order to implement fromNodes :: Nodes -> a, you would need to tell the compiler that a can render any NodeTypes that might be in the Nodes. But since NodeType is extensible, it might be possible Nodes to parse some new FancyNode and Html () not know how to render it (for example).

A clearer type would be to say

data Nodes nodeTypes = ...

where nodeTypes is a type-level set of node types that are represented in a Nodes value, and fromNodes could have a constraint that says "a can render every one of these node types". But this isn't very feasible right now.

The current implementation effectively bundles the Dict for IsBlock (Html ()) (Html ()) when parsing SomeNodeType, so each Node carries around how to "finish" rendering