[w3c/push-api] Need to define where the EventHandler type comes from (#233)

We refer to the EventHandler type in [the Events section](https://w3c.github.io/push-api/#events), which is defined in HTML.

Currently we depend on HTML 5.1 for such definitions, however, it does not export the `EventHandler` type. Instead, it links through to the WHATWG version of the specification, despite defining what it means.

https://www.w3.org/TR/html51/webappapis.html#dom-eventhandlernonnull-event

Is this an oversight? Should we depend on the WHATWG version directly for this type?

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