[pigeon] Reland: Add an initial example app (#3832)
Relands https://github.com/flutter/packages/pull/3761, with the native runners re-created from stable. The important change was the principle class in the macOS Info.plist being NSApplication instead of FlutterApplication, since the latter doesn't exist on stable.
(The pigeon versions also changed because when I re-created the app I re-ran pigeon, and had a slightly newer version.)