Add support for code callouts

[Mpdreamz]

This fixes #88 and properly parses code as regular blocks where its contents are treated as plain text.

Prior to this code blocks behaved like directives and we did a few hacks to get the rendered markdown to display as text.

Had to re-implement supporting substitutions in code blocks since we can no longer rely on SubstitutionBlock being part of its AST.

As for code callouts:

```yaml
project:
  title: MyST Markdown 
  github: https://github.com/jupyter-book/mystmd
  license:
    code: MIT
    content: CC-BY-4.0 <1>
  subject: MyST Markdown 
```

1. The license

A code block can include <\d+> at the end to indicate code callouts. A code block with this style of callouts needs to be followed by an ordered list with an equal amount of items as called out. Otherwise the docs-builder will throw an error.

magic mode

We will also support 'magic' callouts:

```csharp
var apiKey = new ApiKey("<API_KEY>"); // Set up the api key
var client = new ElasticsearchClient("<CLOUD_ID>", apiKey); 
```

Here nothing needs to be numbered and // Set up the api key will be removed from the code block and rendered as the first callout.

This currently works for comments starting with // and # which will cover the majority of languages.

Note the comments have the follow code to be hoisted as a callout.

```csharp
// THIS IS NOT A CALLOUT
var apiKey = new ApiKey("<API_KEY>"); // THIS IS
var client = new ElasticsearchClient("<CLOUD_ID>", apiKey); 
```

This style will allow the callouts to render correctly and for the embedded code to be still syntactically correct.