Move toy examples to "playground" app

[masenf]

We have a lot of cool examples intermingled with some perhaps interesting, but mostly "toy" examples.

This PR creates a single app to contain toy examples, structured in a way that is easy to add onto. Instead of committing a new one page single toy app, add it to the playground instead.

This will allow the other top-level directories in reflex-examples to represent actual complete apps.