[css-values] Spec does not reject duplicate `<request-url-modifier>`s, but WPT does

This test requires that a url() with more than one of the same <request-url-modifier> be treated as invalid, but I can't find anything in the spec that says this is invalid. The 3 major engines all pass these tests, so evidently that's the desired behaviour, but it'd be good to specify it. Correction: The 3 major engines pass that test because they don't support url-modifiers at all. So it's not established whether the test behaviour is what's wanted. 😅

The <url> and <url-modifier> definitions in CSS Values 4 do not place any requirements on repeated modifiers.

Likewise, the definitions in CSS Values 5 of <request-url-modifier>s do not say anything about more than one of a given type appearing in a url().

cc: @weinig as you wrote that particular test.

Start with the CSS Values 4 and definitions, then compare the CSS Values 5 definitions with url-request-modifiers-invalid.sub.html in WPT. Review the discussion and determine whether repeated modifiers should be invalid; done means the relevant specification text clearly resolves that behavior.