Support interactivity in "A Swift Tour"

[amartini51]

The shipping version of A Swift Tour is available as HTML or ePUB like the rest of the book, but also as a downloadable playground to open in Xcode. The code listings were tailored to take advantage of this interactive format, suggesting changes that the reader can make to each code listing to try out parts of the language.

We should extend DocC and adopt that in TSPL, to support some interactive documentation format like a playground. Or in the interim, we could consider reframing the tour to suggest that readers copy/paste each code listing into a playground or REPL, to see the code running and to edit the code.

rdar://71373440