The surrogate code points U+D800 to U+DFFF are valid code points but are not Unicode scalar values. This commit makes the error message more accurately reflect what is actually allowed in `\u` escape sequences.

From https://www.unicode.org/versions/Unicode15.0.0/ch03.pdf:

> D71 High-surrogate code point: A Unicode code point in the range U+D800 to U+DBFF.
> D73 Low-surrogate code point: A Unicode code point in the range U+DC00 to U+DFFF.
>
> 3.9 Unicode Encoding Forms
> D76 Unicode scalar value: Any Unicode code point except high-surrogate and low-surrogate code points.

Related: #20270