Adding support for NSTextInputMarkedRangeAttribute gives macOS accessibility clients knowledge of where input method events are occurring. Without this information, it's impossible for an assistive technology to know if the user is in an input-method composition context.
<rdar://problem/110080851>
Committed 264911@main (94b6fbfe0984): <https://commits.webkit.org/264911@main> Reviewed commits have been landed. Closing PR #14555 and removing active labels.