Implement HTMLMenuElement
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Per spec, "The menu element is simply a semantic alternative to ul to express an unordered list of commands (a "toolbar")." We already have the CSS for it in user-agent.css, and this gives us the interface object.
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- [X] `./mach build -d` does not report any errors
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- [X] These changes fix#24990
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- [X] There are tests for these changes
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