Add support for shared code between chapters

[ashfurrow]

It's as easy as copying Swift files into the Sources folder, we would need to document how playgrounds on the mac need to reference these and not copy them.

[ashfurrow]

This is really tricky, since Playgrounds on macOS will only copy files and not reference them from outside the playground. It may just be a matter of having to duplicate the code across chapters. Not ideal, but might be good enough. Since I haven't had a need to references files across chapters yet, I can't prioritize/figure out the best solution for this issue.

[ashfurrow]

Maybe editing the playground workspace xml manually could be used?

[rhysforyou]

I have a playground book I want to try building over the next week or so that's going to rely on some shared code stuff, so I'll hopefully get a better sense of that stuff.

[ashfurrow]

Cool! I tried symlinks and macOS shortcuts without success.

[dionlarson]

See the discussion on #34

The .playgroundbook format has 3 different Source folders:

• Root Source folder shared by every chapter
• .playgroundchapter/Source folder (similar to .playground/Source)
• .playgroundpage/Source folder (similar to .xcplaygroundpage/Source)

That said, I have not found a way to use the same file in multiple .playground.