Port AbstractHighlighter::highlightLine to QStringView

If your line is already a QString this makes no difference, otherwise this now allows to perform highlighting without the need of splitting lines into QString copies.

The old QString variant has now deprecation warning, as the removal of that will automatically "port" user code. With that overload disabled there is one difference though that can be seen in the test changes: QLatin1String cannot be converted implicitly to a QStringView. For production code this shouldn't matter though.