Auto merge of #21074 - jonathanKingston:toggle-attribute, r=Manishearth

Implement support for Element.toggleAttribute

Copy implementation following from [Firefox implementation](https://bugzilla.mozilla.org/show_bug.cgi?id=1469592) of soon to be standardised [Element.toggleAttribute](https://github.com/whatwg/dom/pull/656). [Other implementers agreed to ship](https://github.com/whatwg/dom/issues/461)

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