Syntax highlight Markdown fenced code blocks

The syntax-highlighting styles are wholly generated from the rouge gem’s Github theme.

To enable it, configure Middleman’s Markdown rendering engine by adding a call to activate :syntax to the project’s config.rb.

To ensure that middleman-syntax and the @tailwindcss/typography styles are not at odds with one another, customize @tailwindcss/typography to omit some of its predefined <pre> element styles.

Append the syntax highlighting CSS to the bottom of source/stylesheets/syntax.css.erb, and extend the Tailwind .prose classes to account for the new .highlight class.