Exposing more elements of `config` to template rendering

The SerializedConfig object that is provided during template rendering does not include all of the elements of the source Config object. In particular, an element that would be useful to a theme I’m working on is exposing the markdown configuration.

I’d use this to allow the theme’s base template to automatically include the syntax-highlighting CSS files, something like this:

{% if config.markdown.highlight_code and config.markdown.highlight_theme == "css" %}
<link rel="stylesheet" type="text/css" href="/syntax-theme-dark.css" media="(prefers-color-scheme: dark)" />
<link rel="stylesheet" type="text/css" href="/syntax-theme-light.css" media="(prefers-color-scheme: light)" />
{% endif %}

which makes Lighthouse happier than using CSS imports and should marginally improve performance.

There’s a workaround, for sure - the site using the theme can be allowed to insert this code using a block.

I’d be happy to work on this and open a PR if the project is open to it.

That would be fine, let’s expose as little as needed though.