Let's add support for interpolating numeric custom properties.
Pull request: https://github.com/WebKit/WebKit/pull/7696
Submitted web-platform-tests pull request: https://github.com/web-platform-tests/wpt/pull/37529
Committed 257930@main (c7b7097190c6): <https://commits.webkit.org/257930@main> Reviewed commits have been landed. Closing PR #7696 and removing active labels.
<rdar://problem/103406770>