[css-values-5] Implement sibling-index() / sibling-count().
This is behind a flag for now, not the least because having such
element-dependent functions available will crash some properties'
parsers, which expect to be able to evaluate calc() expressions
at parse time (for e.g. clamping).
Bug: 40282719
Change-Id: I38d88da8d0492d8725089e4bb3f71944c564a7ac
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5097285
Reviewed-by: Anders Hartvoll Ruud andruud@chromium.org
Commit-Queue: Steinar H Gunderson sesse@chromium.org
Cr-Commit-Position: refs/heads/main@{#1384353}