Auto merge of #20482 - brainlessdeveloper:list-style-image-computed, r=emilio

Implement a URL-generic type for ListStyleImage

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This should fix the following two "expected to fail" tests described in https://github.com/servo/servo/issues/18015:

- getComputedStyle(elem) for url() listStyleImage uses the resolved URL
  and elem.style uses the original URL

- getComputedStyle(elem) for url() listStyle uses the resolved URL
  and elem.style uses the original URL

I updated the test failure expectations by removing the corresponding `.ini` file.

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- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix #18015 (github issue number if applicable).

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