Auto merge of #13066 - mskrzypkows:htmlselect_index, r=KiChjang

Implement indexed access on select elements

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Added methods for indexed access on select: SetLength, Length, Item, IndexedGetter

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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 #11763 (github issue number if applicable).

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- [x] There are tests for these changes OR
- [ ] These changes do not require tests because _____

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components/script/dom/webidls/HTMLSelectElement.webidl

@@ -15,8 +15,8 @@ interface HTMLSelectElement : HTMLElement {
   readonly attribute DOMString type;
 
   //readonly attribute HTMLOptionsCollection options;
-  //         attribute unsigned long length;
-  //getter Element? item(unsigned long index);
+           attribute unsigned long length;
+  getter Element? item(unsigned long index);
   //HTMLOptionElement? namedItem(DOMString name);
   // Note: this function currently only exists for union.html.
   void add((HTMLOptionElement or HTMLOptGroupElement) element, optional (HTMLElement or long)? before = null);