Reduce use of Vector::uncheckedAppend() throughout the codebase.
Pull request: https://github.com/WebKit/WebKit/pull/18931
Committed 269224@main (e7149c4d7559): <https://commits.webkit.org/269224@main> Reviewed commits have been landed. Closing PR #18931 and removing active labels.
<rdar://problem/116831957>