Expose public getter to stylesheet's owner
<!-- Please describe your changes on the following line: -->
- [x] expose a public getter on CSSStyleSheet to return the stylesheet's owner
- [x] in CSSRuleList::insert_rule, use the new getter to pass a non-None value as an argument to arc.insert_rule
- [x] add a test for this that verifies that the new stylesheet is actually loaded
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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#23028
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- [x] There are tests for these changes
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This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/servo/23073)
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