refactor(theme/book): Split into modules (#556)

Advantages:
 - Easier to reason about
 - Can easily disable some modules when debugging
 - Shared dependencies are explicit (playpen_text)
 - Enables some fancier things later one (e.g. run `codeSnippets` slightly later, to avoid blocking the page)

I'm aware that `codeSnippets` should be split into the highlighter and the editor, but I'm not sure I understand exactly how they interact so I've left it as it is for now.

Author: sorin-davidoi