Auto merge of #25413 - pshaughn:optionindex, r=jdm

Implement HTMLOptionElement.index

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Made list_of_options public in HTMLSelectElement so HTMLOptionElement can see it, added index-getting to HTMLOptionElement.

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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 #25392

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- [X] There are tests for these changes OR

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components/script/dom/webidls/HTMLOptionElement.webidl

@@ -22,5 +22,5 @@ interface HTMLOptionElement : HTMLElement {
 
   [CEReactions]
            attribute DOMString text;
-  // readonly attribute long index;
+  readonly attribute long index;
 };