Support `::search-text` (and `:current::search-text`) highlight pseudos from CSS Pseudo-4

[lucinadin]

::search-text is a spec-defined highlight pseudo-element and should be accepted by Lightning CSS parser/selector handling.

Minimal repro:

.x::search-text { color: red; }

Current:

• search-text is treated as unknown/unsupported pseudo-element.

Expected:

• ::search-text parses and serializes as valid selector syntax.
• No unsupported pseudo-element warning for valid syntax.

Why this is valid:

• CSS Pseudo-4 defines highlight pseudos including ::search-text.
• Optional UA implementation does not mean CSS syntax is invalid for tooling.
• Repo precedent for adding highlight pseudos:
  • ::target-text: support ::details-content and ::target-text #888 -> Add support for ::details-content and ::target-text #930
  • ::highlight(): Support for ::highlight() pseudo-element #861 -> Add support for ::highlight #970
  • ::grammar-error / ::spelling-error: Support ::grammar-error and ::spelling-error #1021 -> Add support for ::grammar-error and ::spelling-error #1026

Sources:

• CSS Pseudo-4 highlight pseudos: https://drafts.csswg.org/css-pseudo-4/#highlight-pseudos
• CSS Custom Highlight API context: https://drafts.csswg.org/css-highlight-api-1/