Add UTF-8 case-(in)sensitive Boyer-Moore searcher

For Case-sensitive, we can probably use QByteArrayMatcher.

There's always the obvious algorithm which parses the next MBS into a char32_t and looks up the hash in the skip table. But maybe there's something more clever (not necessarily BM-based).

Researching more clever alternatives is QTBUG-103043. We should at least have peeked into alternatives to the classical BM for UTF-8 before we commit on even an internal API, since alternatives, if they exist, may require a different API approach.