Exposing HTMLSourceElement IDL attributes
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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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Allow supported window attributes to be replacable.
As seen in https://github.com/servo/servo/pull/21045 some Window attributes don't support JavaScript from being able to shadow and replace their values.
This PR adds all other attributes I could find that are supported by servo to be replaceable.
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add TextLength
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add TextLength in HTMLTextAreaElement
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