Auto merge of #25412 - pshaughn:outputvalue, r=jdm

implement HTMLOutputElement.value

HTMLOutputElement now works. I believe layout as well as script is working for it, since it works by putting strings into a child Text node and lays itself out exactly like a span, not requiring any special case rendering like textareas or input elements.

Remaining failures are one mutation test case that everyone else is also failing and probably represents a test/spec mismatch, and all the validation-related tests since we don't have any form-validation hooks.

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