Reject the escape sequences \[uD800] to \[uDFFF] in the parser.

Reject the escape sequences \[uD800] to \[uDFFF] in the parser.
These surrogates are not valid Unicode codepoints,
so treat them just like any other undefined character escapes:
Warn about them and do not produce output.
Issue noticed while talking to stsp@, semarie@, and bentley@.