Add `::search-text` (& `::search-text:current`)

[Schweinepriester]

The ::search-text pseudo-element represents text identified by the user agent's find-in-page feature.

• https://drafts.csswg.org/css-pseudo-4/#selectordef-search-text
• https://github.com/Igalia/explainers/blob/main/css/find-in-page/README.md
• Chrome:
  • https://chromestatus.com/feature/5195073796177920
  • https://issues.chromium.org/issues/339298411
• Firefox: [CSS Pseudo-4] CSS highlight pseudo ::search-text mozilla/standards-positions#1103
• Safari: CSS highlight pseudo ::search-text WebKit/standards-positions#421
• Via
  • https://mastodon.social/@tomayac@toot.cafe/114227557613208897
  • https://blogs.igalia.com/schenney/find-in-page-highlight-styling/