Finish migration to qsizetype [QTextHtmlParser]

Let's be honest: the change from int to qsizetype sizes in Qt 6.0 was more than rushed. Half the API and a good chunk of implementation code is still using int when it should be using qsizetype, and we have been papering over this issue by making qMin() allow mixed-type arguments and, apparently, by removing the annoying MSVC warnings that, in Qt 5, so nicely pin-pointed all these implicit conversions as hard errors.

The parse() method starts by assigning the string.length() to an int variable, so the class is broken for input larger than INT_MAX.