Create a "Hello world" addon

[Duckwhale]

This should serve multiple purposes:

• Provide an example for how addons can be added to the engine
• It's a better new user experience than staring at a blank canvas
• The addons folder is currently not under version control, as it's empty by default
• Can serve as a testing site for demonstrating the core functionality
• May even include the debug tools itself (they're a separate addon currently)