[Cocoa] Hook "prior context" infrastructure up to TextBreakIteratorCF
<rdar://problem/109983296>
*** Bug 257466 has been marked as a duplicate of this bug. ***
Pull request: https://github.com/WebKit/WebKit/pull/14463
Committed 264714@main (7d9c042f3a2b): <https://commits.webkit.org/264714@main> Reviewed commits have been landed. Closing PR #14463 and removing active labels.
This seems to have caused ~5% regression in Speedometer 3's Editor-TipTap suite.