Add `::target-text` (from CSS Pseudo-Elements Module Level 4)

- https://drafts.csswg.org/css-pseudo/#selectordef-target-text
- https://chromestatus.com/feature/5689463273422848
- https://groups.google.com/a/chromium.org/g/blink-dev/c/5zkY3e9cpoo/m/xWNtvgyFAAAJ
- https://twitter.com/tomayac/status/1316671381468258304