MIDIMessageEventInit and MIDIConnectionEventInit should inherit from EventInit

These event inits should inherit from EventInit[1], like other event inits do(e.g. [2]). Otherwise, `bubbles` and `cancelable` attributes aren't initialized correctly.

[1] https://dom.spec.whatwg.org/#interface-event
[2] https://dom.spec.whatwg.org/#customevent