Auto merge of #25449 - pshaughn:select-adding, r=Manishearth

Implement HTMLSelectElement.add() and indexed setter

HTMLSelectElement.add and its indexed setter just needed to forward to matching methods in HTMLOptionsCollection, which they now do. It was also necessary to change codegen slightly; it had accidentally assumed that if an indexed setter existed, a named getter or setter also would.

I expect this and #25446 will combine to pass more tests than either alone does.

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- [X] These changes fix #25003

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