Method reset

src/selection.h:47–53

    [[nodiscard]] inline bool isEmpty() const { return !firstLine.isValid() || (firstLine == lastLine && firstPos == lastPos); }

    void reset()
    {
        oldLastLine = lastLine;
        oldFirstLine = firstLine;
        firstLine.invalidate();
        lastLine.invalidate();
    }

    void start(LineRef l, qsizetype p)
    {