Hook "prior context" infrastructure up to TextBreakIteratorICU
<rdar://problem/109989795>
Pull request: https://github.com/WebKit/WebKit/pull/14466
Is this also a dupe of bug 257467?
Committed 264722@main (b42c0d74cd2e): <https://commits.webkit.org/264722@main> Reviewed commits have been landed. Closing PR #14466 and removing active labels.