Auto merge of #24680 - saschanaz:formdataevent, r=jdm

Require FormDataEventInit dictionary

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Should probably wait until https://github.com/whatwg/html/issues/5064 be fixed.

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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 __ (#22670 mentions `FormDataEvent` but the codegen itself still has a problem, in e.g. `PromiseRejectionEventInit`)

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- [x] There are tests for these changes

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tests/wpt/metadata/html/dom/idlharness.https.html.ini

@@ -1409,9 +1409,6 @@
   [SubmitEvent interface: existence and properties of interface prototype object's "constructor" property]
     expected: FAIL
 
-  [FormDataEvent interface object length]
-    expected: FAIL
-
 
 [idlharness.https.html?include=(Document|Window)]
   [Document interface: documentWithHandlers must inherit property "queryCommandEnabled(DOMString)" with the proper type]