Auto merge of #22648 - collares:ChannelSplitterNode, r=Manishearth

Implement DOM APIs for ChannelSplitterNode

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Based on #21591. Fixes #21558. I tried to update the expected results for WPT using "./mach update-wpt"; let me know if I got something wrong.

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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 the second half of #21558

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- [x] There are web-platform-tests tests for these changes
- [ ] These changes do not require tests because ___

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