Breaking API change: Export promise

Instead of having to manually call `puppet.exit()`, make it a convention-based API:

Each JS/TS entrypoint must `export default` a promise or a function returning a promise. When this promise resolves, the script is considered to be finished.

Allow manual dynamic tasks by calling `puppet.run(functionOrPromise)`.

If you export / run a function, we can pass the original shell `argv` and runtime data created by plugins.

For your convenience:
Consider that the script might be a CommonJS module, not an ES module. Also gracefully deal with entrypoint functions that don't return a promise.