Remove onEvent and add subscription noun instead

[sam-goodwin]

We've decided to apply the following convention across all Eventual primitives.

noun(fqn, props, handler)

For example:

activity("activityName", { timeout: duration(1, "minute") }, async () => {
  await database.get()
});

As a part of #264, we've also landed on the following name:

commands // all of the named commands/apis 
subscriptions // all of the named subscriptions
events // of the named event schemas
activities // all of the named activities
workflows // named workflows
// etc.

This issue tracks the change to remove onEvent and instead add the subscription entity:

export const onUser = subscription("onUser", {
  events: [OnUserEvent]
}, async (event) => {
  //
});

Benefits:

1. it is consistent and aligns with all other eventual primitives
2. it reads better - userEvent.onEvent("onUserEvent" is flawed because it reads like the name is the message being subscribed to.
3. properties can be used for declarative configuration