[anchor] Implement tree-scoped 'all' for anchor-scope
This introduces tree-scoped keywords (not custom idents) for the first
time. To not waste memory, we keep CSSIdentifierValue as a tree-scope-
agnostic value, and instead use the new CSSScopedKeywordValue whenever
a tree-scoped keyword is needed.
Fixed: 368034380
Change-Id: Ib64d1acaf660e2ce6a1357be3cbe8a62e3655978
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5872877
Reviewed-by: Rune Lillesveen futhark@chromium.org
Commit-Queue: Anders Hartvoll Ruud andruud@chromium.org
Cr-Commit-Position: refs/heads/main@{#1358734}