Auto merge of #25627 - pshaughn:definedpseudo, r=emilio,jdm

Implement :defined CSS selector

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ElementState::IN_DEFINED_STATE already existed but wasn't hooked up to anything; now it is set appropriately, and CSS is aware of it. The main test for this selector needed the window named getter for no important reason, so I made a one-line test change to avoid that. Remaining failures in the test are all about SVG elements.

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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 #25010

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- [X] There are tests for these changes OR
- [ ] These changes do not require tests because ___

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