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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- [ ] These changes don't fix an issue however the wpt-tests will merge soon and will cause an issue etc.
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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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Implement window.length DOM attribute.
Implements the `window.length` DOM attribute for enumerating frames on the document.
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Implement read methods on FileReaderSync
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Implemented the read methods on the FileReaderSync struct according to https://w3c.github.io/FileAPI/#dfn-FileReaderSync
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- [x] These changes fix#15114
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Implement MutationObserver.disconnect()
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This implements https://dom.spec.whatwg.org/#dom-mutationobserver-disconnect
I added a `node_list` to the `MutationObserver` struct to keep track of the nodes involved in an observer because otherwise I could not find a way to unregister the observers on the node themselves without traversing the whole document tree. Let me know if there's something I missed.
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Implement beforeunload event and infrastructure
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Implementation of:
1. https://html.spec.whatwg.org/multipage/#prompt-to-unload-a-document, and
2. https://html.spec.whatwg.org/multipage/#unload-a-document
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- [ ] These changes fix#10787 and fix#20485 and fix#20588 and fix#20496 (github issue number if applicable).
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