[css‑syntax] `<string‑token>` should probably handle valid surrogate pairs

Spec: https://drafts.csswg.org/css-syntax-3/#consume-string-token

The 6 digit syntax for <string‑token> was only added to CSS in CSS2, with the CSS Syntax module restricting it to not allow surrogate code points, but I would expect that a string containing valid surrogate pairs would work like the equivalent 6 digit syntax, e.g.:

.foo:before {
	/*
	The "CSS1" and "CSS2.1" specifications parse this as:
		- U+D83D (High Surrogate)
		- U+DD25 (Low Surrogate)

	The "CSS Syntax Level 3" specification parses this as:
		- U+FFFD (Replacement Character)
		- U+FFFD (Replacement Character)

	I would expect this to parse as the U+D83D and U+DD25 surrogate code points,
	which would decode to U+1F525 (Fire) at parse time.
	*/
	content: "\D83D\DD25";
}

and

.foo:before {
	/*
	The "CSS1" specification parses this as:
		- U+1F52 (Greek Small Letter Upsilon With Psili And Varia)
		- U+35 (Digit Five)
	The "CSS2.1" and "CSS Syntax Level 3" specifications parse this as:
		- U+1F525 (Fire)
	*/
	content: "\1F525";
}

would be equivalent.

Start by reading the linked CSS Syntax Level 3 section on consuming string tokens and compare its treatment of surrogate code points with the examples in the issue. The issue asks whether valid surrogate pairs should be handled differently; done would require a resolved specification decision and any corresponding draft change, but no implementation files or tests are named.