[w3c/uievents] Specify the arguments of initCompositionEvent() (#134)

`initCompositionEvent()` is supported in Blink, EdgeHTML, Gecko and WebKit. Usage is [very low](https://www.chromestatus.com/metrics/feature/timeline/popularity/863) but it's still not a good use of time trying to remove it from all engines.

Instead, specify it like this:
```WebIDL
void initCompositionEvent(DOMString type,
                          optional boolean bubbles = false,
                          optional boolean cancelable = false,
                          optional Window? view = null,
                          optional DOMString data = "");
```

See also https://github.com/whatwg/dom/issues/387

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