Auto merge of #22917 - ksqsf:master, r=KiChjang

Add optional annotations for initEvent method of Event interface

Added `optional` and default values to parameters `bubbles` and `cancelable` of `initEvent` of `Event`.

I tried to update test results, but there seem to be no differences at all.

I checked similar code where `optional` is used, and there seems to be no special handling in Rust. If that is not the case with this issue, please let me know and I'll commit more follow-ups.

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- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix #22883

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- [x] There are tests for these changes OR

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components/script/dom/webidls/Event.webidl

@@ -35,7 +35,7 @@ interface Event {
   [Constant]
   readonly attribute DOMTimeStamp timeStamp;
 
-  void initEvent(DOMString type, boolean bubbles, boolean cancelable);
+  void initEvent(DOMString type, optional boolean bubbles = false, optional boolean cancelable = false);
 };
 
 dictionary EventInit {