Create .onAny and .onError methods, deprecate "*" and "error" events

[gr2m]

Follow up to #180

I would reserve the .on(event, handler) method for actual events only, and define new methods for special events that are unique to @octokit/webhooks.

That will also address #302, because we would no longer need to define * and error as part of the overall event map

[ankeetmaini]

I like this approach, as due to this all other types collasce to any and we lose the strong typing. I think with this, we can event type the hook functions as Handler<E, U> instead of plain Function

[github-actions]

🎉 This issue has been resolved in version 7.15.0 🎉

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