Auto merge of #20329 - gterzian:before_unload, r=cbrewster

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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- [ ] `./mach build -d` does not report any errors
- [ ] `./mach test-tidy` does not report any errors
- [ ] These changes fix #10787 and fix #20485 and fix #20588 and fix #20496 (github issue number if applicable).

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