Update HTMLFastPathParser::scanText() to return a String instead of a `std::variant<Span<const LChar>, Span<const UChar>>`.
Pull request: https://github.com/WebKit/WebKit/pull/12354
Committed 262564@main (e947c7c9fe38): <https://commits.webkit.org/262564@main> Reviewed commits have been landed. Closing PR #12354 and removing active labels.
<rdar://problem/107595915>