With the introduction of the cascade layer, the 5th CSS-wide keyword,
`revert-layer`, has been added.
(cherry picked from commit bea7eec5183a816a100d190e409a622f429d7405)
For a long time, we've used two terms, inconsistently:
- "Identifier" is a spec term, but refers to a sequence of alphanumeric
characters, which may or may not be a keyword. (Keywords are a
subset of all identifiers.)
- "ValueID" is entirely non-spec, and is directly called a "keyword" in
the CSS specs.
So to avoid confusion as much as possible, let's align with the spec
terminology. I've attempted to change variable names as well, but
obviously we use Keywords in a lot of places in LibWeb and so I may
have missed some.
One exception is that I've not renamed "valid-identifiers" in
Properties.json... I'd like to combine that and the "valid-types" array
together eventually, so there's no benefit to doing an extra rename
now.
(cherry picked from commit 6a74b0164423d63904cf5a5b594772b595f57600;
very minorly amended to fix conflict in GenerateCSSKeyword.cpp caused
by #22870, and in libweb_generators.cmake due to us not having
https://github.com/LadybirdBrowser/ladybird/pull/741)
