[css-properties-values-api-1] Define `syntax` with `<string> | <syntax>`

[cdoublev]

The syntax descriptor could now be defined with a "naked" value matching <syntax>.

When declared as a <string>, its value would be parsed against <syntax> instead of calling consume a syntax definition, which could be removed.

Note that the <syntax> value range is slightly wider than what this algo allows, since it matches syntax component names case-sensitively. I think that is the only difference.