The definition of syntax in the "css-properties-values-api" spec (which
is used for the `@property/syntax` descriptor) is slightly different
from the definition of `<syntax>` in the "css-values" spec (which we
implement) in that it limits literal idents to exclusively
`<custom-ident>`s (i.e. not CSS-wide keywords or "default").
`<custom-ident>`s are also case-sensitive so that behavior is
implemented for syntax matching here as well
This reverts 0e3487b9ab.
Back when I made that change, I thought we could make our StyleValue
classes match the typed-om definitions directly. However, they have
different requirements. Typed-om types need to be mutable and GCed,
whereas StyleValues are immutable and ideally wouldn't require a JS VM.
While I was already making such a cataclysmic change, I've moved it into
the StyleValues directory, because it *not* being there has bothered me
for a long time. 😅
`<syntax>` is a limited subset of the "value definition syntax" used in
CSS specs. It's used for `@property`'s `syntax` descriptor, and for the
`type()` function in `attr()`.
