[css-properties-values-api] Allow dashed idents in syntax strings

[andruud]

Currently, grammars like syntax:"auto | --mything" are not allowed, since we don't handle HYPHEN-MINUS during "consume a syntax component". This was likely an oversight when it was initially written.

Note: we might want to just remove the algorithms that parse syntax components from a string, and instead tokenize/parse using the new <syntax> production. If we do, that would also make dashed idents allowed unless we explicitly block it.