Auto merge of #17614 - cbrewster:custom_element_reactions, r=jdm

Implement custom element reactions

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Initial work for implementing custom element reactions: https://html.spec.whatwg.org/multipage/custom-elements.html#custom-element-reactions

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- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #17433 (github issue number if applicable).

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