When including a CSS file in a HTML file sent to QTextDocument, and the
CSS file starts with «@charset "UTF-8";», which is the correct way of
declaring that, the parsing fails.
If you omit the space, like «@charset"UTF-8";» the parsing succeeds,
which is wrong.
Fix this by expecting and swallowing whitespace after the @charset
tokens.
Task-number: QTBUG-54829
Change-Id: I32044e8d24bda70c1eb06bf74af50d4cabe2211d
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>