Add a notifications API for triggering the send of notifications via email, SMS, etc.

[rglover]

I always rewrite this on projects and it'd be helpful to just have a pattern. Something like this:

import joystick from '@joystick.js/node';

joystick.app({
  notifications: {
    providers: {
       email: {},
       sms: {},
    },
  },
});

Another one of those "think about it" problems because we want to avoid bogging down the package with dependencies.