[css-pseudo-4] Add a new pseudo element search-text

[jihyerish]

Add a new pseudo-element ::search-text which is highlight pseudo-elements allowing CSS styling of the search results shown using a browser’s find-in-page feature and its painting order among others.

There are resolutions from CSSWG:

• Add ::search-text : https://github.com/w3c/csswg-drafts/issues/10212#issuecomment-2089302563
• Painting order of ::search-text: https://github.com/w3c/csswg-drafts/issues/10213#issuecomment-2089311110

[jihyerish]

@fantasai , In the resolution , this feature needs to be marked as optional to implement. How can I put it in the spec?

[fantasai]

I rebased the PR to clear the conflicts and made some edits and merged it, so closing.