Add support for setup and install events for web extensions.
<rdar://problem/103361287>
rdar://103261455
Pull request: https://github.com/WebKit/WebKit/pull/18660
Committed 268932@main (c3d35ed19457): <https://commits.webkit.org/268932@main> Reviewed commits have been landed. Closing PR #18660 and removing active labels.