refactor: introduce shared Context to replace parent in Runner.__init__()

[nats]

• Introduce new Context class, which replaces parent Runner as mandatory init argument
• At runtime, a single instance of Context will be provided to all runners.
• Move Runner.prog to Context
• Rename App to AppRunner, and add new App that creates the Context and initialises AppRunner

In future, Context will be used to provide shared access to "program state" - configuration settings, available commands, etc.