Auto merge of #11726 - sjmelia:7720_add_target_selector, r=mbrubeck

Issue 7720: Add target selector and update when scrolling to fragment

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Add the target pseudo selector and set/unset it during scrolling to fragment. This change is not complete as no repaint is triggered after the selector is added - it will only take effect after a repaint is triggered by e.g. hovering over another element. (See manual test)

I would like some help because i'm not sure how to resolve this; I can only think to call window.reflow.

I added a manual test case, don't think this counts really! I think the applicable automated test is in /tests/wpt/web-platform-tests/dom/nodes/Element-matches.html but it currently fails, I think due to the above.

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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 #7720  (github issue number if applicable).

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