vale: exempt code snippets from Qt.Mechanics.LineLength

The 80-column limit applies to documentation prose. Code follows the Qt
coding style instead. LineLength.tengo skipped code outside /*! blocks
but still measured snippet lines inside \code, \badcode, and \qml
blocks, so a long line in an example snippet raised a warning.

Track the open snippet block and skip its lines until the matching
\endcode or \endqml. The lines holding the opener and the closer are
still measured, because the rest of each line is QDoc markup.

Add tests for a long line in each block type, and for a long prose
line after a block closes.

Change-Id: I5fc2eb344561ec82fad07147e353d8234a9187d0
Reviewed-by: Venugopal Shivashankar <Venugopal.Shivashankar@qt.io>
Reviewed-by: Alexei Cazacov <alexei.cazacov@qt.io>